Local Area Traffic Management
Table of Contents
- 1. INTRODUCTION
- 2. ROUNDABOUTS
- 3. THRESHOLD ENTRY STATEMENTS
- 4. ROAD HUMPS
- 5. HORIZONTAL DISPLACEMENT DEVICES (SLOW POINTS)
- 6. DRIVEWAY LINKS
- 7. MODIFIED "T" INTERSECTIONS
- 7.1 MODIFIED "T" INTERSECTIONS - GENERAL
- 7.2 MODIFIED "T" INTERSECTIONS - DESIGN CONSIDERATIONS
- 7.3 MODIFIED "T" INTERSECTIONS - ADVANTAGES
- 7.4 MODIFIED "T" INTERSECTIONS - DISADVANTAGES
- 7.5 MODIFIED "T" INTERSECTIONS - MAY CONSIDER USINGWhere there is a need to regulate traffic movements.
- 7.6 MODIFIED "T" INTERSECTIONS - WHEN NOT TO USE
- 7.7 MODIFIED "T" INTERSECTIONS - Change of Stop or Give Way Priority at Four-Way and 'T' Intersections
- 8. BLISTER ISLANDS
- 9. PARTIAL CLOSURES
- 11. MEDIAN TREATMENTS
- 12. STANDARD DESIGN ENVELOPES
- 13. DRAWINGS
These Guidelines set out the recommended geometry and layout of signs and lines for a number of common traffic management devices in use in Western Australia. The guidelines should be read in conjunction with other reference material such as the Austroads Guide to Traffic Engineering Practice series, with particular reference to Part 10 - "Local Area Traffic Management", Main Roads Technical Design Criteria, the AS 1742 series of Standards and the Bikewest Guidelines for the Design of Bicycle Facilities. The document for the Main Roads WA course "Cycling Infrastructure Course" also provides guidance on good traffic engineering practice with respect to cycling facilities.
The guidelines are not limited to consideration of traffic control devices in the Perth Metropolitan area, but have application in both rural and country-urban situations.
These guidelines, along with the material referred to above, supersede the document "Guidelines for Local Area Traffic Management", Pak-Poy and Kneebone for Main Roads Department Western Australia, 1990.
1.2. OBJECTIVES OF THE GUIDELINES
The objectives of the guidelines are to provide a resource document that reflects best practice, allowing practitioners to develop meaningful traffic management schemes that:
- Are uniform within the State.
- Meet Main Roads standards for signs and linemarking
- Are predictable for road users moving from one area of Authority to another.
- Create safer road environments thereby reducing the community trauma associated with crashes.
- Improve the local amenity of residential precincts.
The Guidelines are intended to provide an overview of Traffic Control Devices for all interested persons while guiding qualified and experienced traffic engineering personnel in the development of detailed design for geometric elements and signs and lines. They are not intended to replace professional judgment and ability.
1.3. COMPLIANCE WITH THE GUIDELINES
Whilst there is no compulsion to comply with the recommendations of these Guidelines except in those areas where Main Roads has final responsibility or legislative and statutory requirements exist, practitioners are urged to carefully consider the objectives of these Guidelines before proceeding with alternative practices. The erection, modification or removal of traffic control devices involves changing the road environment and this requires practitioners to meet any applicable statutory obligations and the common law "duty of care".
These Guidelines have been developed following a review of national and local practices and incorporate 'best practice' principles from both these sources.
The objective in producing these Guidelines should not be seen as a deterrent to innovation in the design of traffic management devices. The consistency sought is in the application of uniform design principles, in the signing and linemarking of devices and in taking a common approach to the continuous improvement of the road environment.
There exists a large amount of reference material pertaining to the various aspects of Traffic Control Devices including Design Guidelines and Australian Standards. It is not intended that these LATM Guidelines replace or restate existing industry accepted standards, guidelines or practices.
Guide to Traffic Engineering Practice series, with particular reference to Part 10 - "Local Area Traffic Management", Austroads, 2004
|AS 1428.4||Design for Access and Mobility - Tactile Ground Surface indicators for the Orientation of People with Vision Impairment|
|AS1742||Manual of Uniform Traffic Control Devices (Parts 1 to 15) (From time to time individual documents are revised, which may change or include signs that are not included in AS1742.1. Therefore all documents relevant to a particular type of traffic control device should be referenced for advice.)|
|AS1743||Road Signs - Specifications|
|AS1906.1||Retroreflective materials and devices for road traffic control purposes Part 1: Retroreflective materials|
Main Roads WA
Main Roads Specification 601: Signs
Main Roads Disability Access and Inclusion Plan 2007-2011
"Cycling Infrastructure Course", Main Roads WA
Main Roads Guidelines:
- Sign Standards
- Pedestrian / Cyclist Facilities
- Sign Structural Design Guidelines
- Main Roads Signs Index
"Guidelines for the Design of Bicycle Facilities", Bikewest Western Australia, Department of Transport
1.5. GENERAL PRINCIPLES
When designing traffic control devices, consideration should be given to the functional classification of the road/roads as well as:
- The type of vehicles expected to use the route;
- The needs and safety of cyclists, pedestrians and people with disabilities;
- Public Transport usage;
- Access requirements of emergency and service vehicles; and
- Property access and the street amenity
Generally, the following issues should be addressed in the design of LATM devices.
- All physical devices should be designed to minimise injury to drivers and their passengers etc.
- All physical devices should be designed in such a manner so as to minimise damage to vehicles failing to negotiate them in a correct manner.
- Semi mountable kerbing, frangible poles, bollards, signs and vegetation should be used.
- Power poles should be relocated away from kerb edges particularly on departure sides of devices.
- Where landscaping is used as an adjunct to physical devices either to increase the effectiveness of the device or to improve local amenity, sight lines should not be reduced to unsafe levels.
- Pedestrian crossing sight distance for all pedestrians including small children and people with disabilities (e.g. wheelchair bound persons) should be maintained at all crossing points.
- Where LATM devices are installed, lighting should be in accordance with Main Roads Lighting guidelines. The impact of proposed landscaping on the effectiveness of street lighting should also be considered at the design stage.
- Landscaping should create visual continuity, reinforce the local nature of the road environment and improve local amenity.
- Landscaping should be used to reinforce vehicle and pedestrian paths without obscuring visibility.
- Appropriate plants should be selected for;
- Their shape or form
- The existing vegetation character
- The existing site characteristics
- The landscape character of the area and
- The maintenance regime that will apply
- All design should be based on the appropriate turning templates. However in the absence of a specified larger vehicle, the minimum design vehicle should be the 8.8m service vehicle (refer to Austroads design vehicles and turning path templates guide). Consultation with Transperth as part of the planning stage of developing LATM treatments is essential.
- Designs should take into consideration public transport requirements as well as the special needs for schools, tourist operations etc.
- All walkways and pedestrian ramps should be designed to the Main Roads' document, Main Roads Disability Access and Inclusion Plan 2007-2011 and Main Roads WA drawing number 9831-5649.
- The installation of Tactile Ground Surface Indicators (TGSI's) shall be in accordance with AS1428.4. Most importantly, the TGSI's should be aligned in the direction of travel and span the width of the ramps (100mm gap on the edge). (Refer to photographs).
Median Gap - "Cut through" with TGSI's
Kerb ramp aligned in direction of travel. TGSI's in accordance with AS1428.4
LATM devices can improve the overall safety and amenity for cyclists by reducing traffic speeds and volumes. However, individual devices can obstruct bicycles or increase the potential for conflict between bicycles and vehicles. The design of LATM devices should consider the following, particularly on routes with a high bicycle patronage, or on routes that are likely to attract high bicycle patronage as a result of future development or as a result of the provision of safer cycling facilities:
- Vertical displacement devices are considered safer and more acceptable by cyclists than horizontal displacement devices.
- The creation of "squeeze points", whereby the road is narrowed without a corresponding adequate speed reduction should be avoided. Specific attention should be given to the safety of cyclists under these circumstances and consideration should be given to providing a "by-pass" for cyclists.
- Lane widths should be 3.0m or less or 3.7m or greater so that there is only room enough for a cyclist to occupy the lane as a single vehicle, or there is sufficient room for a cycle and a vehicle to occupy the lane side by side.
- The provision of parking near LATM devices should ensure that entry and exit to and from the device is not obstructed.
- It is essential that LATM devices provide for cyclist safety and accessibility where they are installed on designated cycling routes such as Local Bicycle Routes as part of the Perth Bicycle Network. Designers should also consider the need to provide for cyclists' safety and accessibility at LATM devices, which are installed on roads that are not designated cycling routes.
- Installation of the LATM treatments in isolation may merely serve to shift traffic to adjoining streets. The expected traffic impacts on the adjoining neighbourhood area should therefore be considered at the planning stage.
- U-turn, or other unsafe manoeuvres, resulting from the closure or partial closure of local roads, should also be considered as part of the planning and design process.
Local road roundabout - see drawing 200831-0016
Mini roundabout - see AS1742.13
2.1 ROUNDABOUTS - GENERAL
A well designed roundabout is a safe and effective form of intersection control that can be installed on both four legged and three legged intersections. Roundabouts have been used extensively in Europe, the UK and in Australia, and are generally found by most road users to be one of the more 'acceptable' forms of traffic control. However, roundabouts do present problems for cyclists, pedestrians and people with disabilities and should be avoided at high pedestrian usage intersections such as those near schools, unless appropriate measures to accommodate vulnerable road users (e.g. use of school crossing wardens) are implemented. Roundabouts reduce the relative speeds of conflicting vehicles by providing impedance to all vehicles entering the roundabout. The safety performance of roundabouts is well proven and if designed and applied correctly, a roundabout can be expected to be better than other forms of channelisation. Whilst roundabouts may not reduce overall crash numbers at a location, they generally reduce the severity.
2.2 ROUNDABOUTS - DESIGN CONSIDERATIONS
When designing a roundabout, consideration should be given to the functional classification of the intersecting roads as well as:
- The type of vehicles expected to use the intersection.
- The needs and safety of cyclists and pedestrians crossing the intersection, and
- Access requirements of emergency and service vehicles.
The Austroads publications "Guide to Traffic Engineering Practice, Part 6 - Roundabouts and Part 14 - Bicycles" and the Main Roads Roundabouts Design Guidelines should be referred to for further information regarding this type of treatment.
When designing geometric elements and signage and linemarking for roundabouts, the following should be considered;
- Ensure adequate deflection on the entries to the roundabout to encourage low approach speeds. The entry design speed should be similar or less than the exit design speed.
- Pedestrian gaps should be located desirably 6.0m back from the Give Way line to enable a pedestrian to cross behind a vehicle stopped on the approach.
- Where supervised school crossings are to be installed, pedestrian gaps should be located desirably 12.0m back from the Give Way line.
- Pedestrian ramps are to be aligned with the splitter island "cut throughs".
- Tactile Ground Surface Indicators (TGSI's) should align with the direction of travel and drawings should accurately depict this.
- Circulating widths need to be designed to accommodate all design vehicles.
- On bus routes, turning paths should be designed to accommodate a Mercedes 405 bus type. The template for the swept path shall be the vehicle overhang path.
- On bus routes, circulating width should be between 6.0 and 7.0 metres, with the inner annulus radius being between 6.5 (minimum) and 7.5 (desirable) metres.
- On bus routes, all kerbing and pavement should be designed to provide clearance for bus overhangs. Consideration needs to be given to height clearances of low floor buses.
- Advance warning signs are not mandatory and should only be used to address specific needs.
- Chevron signs in the annulus are not mandatory and should only be used to address specific needs.
- Where "nibs" are included in the design of approaches to roundabouts, hazard markers (D4-1-2) should be installed on the nibs.
- There are significant potential dangers for cyclists and pedestrians at roundabouts if they are not appropriately designed. Careful consideration of cyclist and pedestrian needs should be an integral part of the design process.
- Where roundabouts occur on cycle routes, consideration needs to be given to providing cycle route signs and pavement markings.
- The maximum height of landscaped material should be in accordance with Austroads design Guidelines. All landscaping material should be frangible.
- Where high pedestrian and cycling activity is present, high quality shared-paths should be provided as shown in Austroads Part 14 - Bicycles. (See also photograph below).
Alternative cycling provision - off-road shared path at roundabout
2.3 ROUNDABOUTS - ADVANTAGES
- Reduces the severity of crashes.
- Controls traffic movement.
- Lowers speed.
- Can be landscaped.
- Low maintenance.
2.4 ROUNDABOUTS - DISADVANTAGES
- Loss of kerbside parking
- Squeeze point for cyclists
- Relatively expensive device
- Is not friendly towards pedestrians or people with disabilities
2.5 ROUNDABOUTS - MAY CONSIDER USING
Roundabouts can be used satisfactorily at a wide range of sites, although they perform better at the intersection of roads with roughly similar traffic flows.
- Intersections on urban Local and Local Distributor roads.
- Intersections on Primary and District Distributor roads in urban areas.
- Intersections on local rural roads where these are sealed.
2.6 ROUNDABOUTS - WHEN NOT TO USE
- Where marked uneven flows of traffic occur.
- Where satisfactory geometry cannot be provided due to insufficient space or other constraints. ie proper deflection cannot be achieved to reduce speeds.
- Where large combination vehicles or over dimensional vehicles frequently use the intersection.
- At an isolated intersection in a network of linked signals.
- Adjacent to crossing points (school, railway etc.) where queuing may back up through the roundabout.
- Over a crest.
3. THRESHOLD ENTRY STATEMENTS
Raised entry statement refer drawing 200331-0127
3.1 THRESHOLD ENTRY STATEMENTS - GENERAL
Entry Statements are designed to provide visual and tactile clues to drivers, alerting them that they are entering a driving environment that is different from the one they have just left. They are commonly used at the junction of Access roads and Distributor roads particularly as part of LATM schemes. They may be constructed flush with the surrounding pavement or incorporate a raised plateau section. Entry Statements are also commonly used at the boundaries of differing land uses, for example at the interface of residential and commercial properties or on either side of a school. Entry statements may also be used mid-block to designate a change in speed environment.
3.2 THRESHOLD ENTRY STATEMENTS - DESIGN CONSIDERATIONS
When designing Entry Statements, the following principles should be borne in mind:
- Where median islands are included, lane widths should provide for the turning movements of all vehicles including service vehicles.
- Appropriate parking prohibitions should be provided for safe approach to, and departure from the device.
- Median design should provide adequate width, openings and geometry for pedestrian crossings including making provision for bicycles and gophers.
- Entry statements should not be constructed from the same coloured material as the adjacent footpath, especially if the statement incorporates a plateau, as this gives the false impression of a formal pedestrian crossing facility;
- Entry Statements often rely on different coloured and/or different textured pavement materials such as coloured asphalt, brick or concrete paving. They may also incorporate raised plateaux, central medians or 'nib' carriageway narrowings. Vertical displacement devices are considered safer and more acceptable by cyclists than horizontal displacement devices.
- Where Entry Statements are located midblock, their locations should be selected to maintain resident's property access wherever possible.
- Lane widths should be sufficient to allow cyclists and vehicles to pass and not create a 'squeeze' point, or provide an alternative adjacent cycle facility.
- The choice of pavement material should avoid low skid resistance or heavily textured materials that may be a potential hazard to cyclists and should have similar skid resistance to adjoining pavements.
- Designers should also be aware that the use of different pavement materials such as concrete or brick paving can lead to localised depressions due to pavement failure, which may also be a potential hazard to cyclists. (See photograph below).
Localised depression at the interface between two different pavement materials, resulting in a potential cycling hazard
- Entry statements on bus routes should not incorporate a raised plateau.
- Entry Statements incorporating a raised plateau should be located at least 6.0 metres from the intersection through kerbline.
- Where entry statements incorporate a raised plateau, the design should ensure that there is no conflict between the plateau and pedestrian crossing points.
- Where plateaux are on cycle routes ramp grades shall be modified to 1 in 20 and the effective lane width (lane width less drainage channel and plateau side slope) shall be sufficient to permit a car and bicycle travelling abreast. If this cannot be achieved, a suitable bypass shall be provided. At a minimum, this shall consist of a large enough gap at the kerbside to facilitate the passage of cyclists and nearby parking restrictions shall provide for effective use of the gap by cyclists (refer to Main Roads Drawing Number 200331-0127).
- Where at grade plateaus are used, they should be located at least 1.0m back from the intersection through kerbline to allow holding lines to be marked on an asphalt surface.
- If line marking is to be applied, then coloured asphalt is preferred over concrete or brick paving.
3.3 THRESHOLD ENTRY STATEMENTS - ADVANTAGES
- Alerts drivers that they are entering a different driving environment.
- Reduces approach speeds to an intersection or a lower speed zone.
- Separates residential areas from areas of non residential use.
- Highlights the presence of an intersection or a lower speed zone
3.4 THRESHOLD ENTRY STATEMENTS - DISADVANTAGES
- Effectiveness may be limited if not used in association with other devices.
- Low speed turns from Distributor roads may affect traffic flow or result in an increase in rear end crashes.
- Has little traffic management effect if flush with no centre medians.
- Some localised vehicle noise with raised and textured pavements.
- Some vertical acceleration if used with raised pavements.
3.5 THRESHOLD ENTRY STATEMENTS - MAY CONSIDER USING
- Where there is a need to reduce the capacity of an intersection as part of a LATM scheme.
- At boundaries between different classifications of streets.
- At boundaries between different land uses.
- At boundaries of local area speed Limits.
3.6 THRESHOLD ENTRY STATEMENTS - WHEN NOT TO USE
- At the junction of two Access roads, unless one of the access roads has a markedly different speed environment.
- On wide carriageways unless road narrowing is provided.
- On roads with more than 4,000 VPD.
4. ROAD HUMPS
Watt's profile see drawing 200331-128
Plateau hump see drawing 200331-129
Wombat crossing see drawing 200631-0001
4.1 ROAD HUMPS - GENERAL
Road humps are used to moderate vehicle speeds by the introduction of vertical displacement either in the form of a raised curved section constructed across the line of a carriageway or a raised plateau structure. They are normally located on access roads, which are residential in nature and have only very limited use on Distributor status roads. Frequently they are used in series along a street to maintain low vehicle speeds.
Road humps may also be used in conjunction with a marked pedestrian crossing to form a "Wombat crossing" where pedestrian crossings are needed and warrants are met. Road humps may not be favoured by residents outside whose properties they are constructed due to an increase in vehicle noise.
Bus operators or emergency service agencies, particularly the Ambulance Service, do not favour road humps.
4.2 ROAD HUMPS - DESIGN CONSIDERATIONS
For road humps to operate efficiently and safely, the following design principles should be adhered to:
- The first hump should be within 50 metres of the start of the street where approach speed is low. If necessary, a treatment such as an Entry Statement may be provided to regulate approach speeds.
- Sight Distance commensurate with an operating speed of 50 km/hr (or the prevailing speed zone in the street) should be available to approaching motorists.
- Humps should be constructed at right angles to the direction of travel.
- Where used in series, hump spacing should ideally be between 80 and 100 metres with a maximum of 200m.
- Humps should extend laterally across the full width of the road pavement for all Wombat crossings. Elsewhere humps should extend laterally across the full width of the road except to allow for drainage and cycle by-pass facilities.
- Humps should be clear of driveway entrances to avoid ground clearance problems when vehicles are accessing driveways.
- If a plateau type road hump is located at a Wombat crossing or a signalised Pelican crossing, the crossing should be made a formal crossing with cross walk markings. In these cases pedestrian crossing warrants must be met.
- Where humps are on significant cycle routes, ramp grades should be modified to 1 in 20. If this cannot be achieved, a suitable bypass shall be provided, except at Wombat crossings. As a minimum, this shall consist of a sufficiently large gap at the kerbside to facilitate the passage of cyclists and nearby parking restrictions shall provide for the effective use of the gap by cyclists. The preferred option is to modify the ramp grade since vehicles may use the gap to bypass the ramp thus reducing its effectiveness in slowing traffic down.
- Piano markings on humps are to be located on the approach ramp and not on the road surface immediately before the ramp.
- Ramp grades are to be designed so that they are differential to the road surface and not related to a horizontal datum.
- On bus routes, hump height should be between 65 and 75 millimetres.
- On bus routes and cycle routes, ramp grades should be 1 in 20.
- The designer should ensure that the road drains efficiently.
- The portion of the hump on which the "piano" markings are to be applied, should be constructed from asphalt.
4.3 ROAD HUMPS - ADVANTAGES
- Slows vehicles to about 20 - 25 km/hr. at the device.
- When used in series reduces vehicle speeds along the entire length of the street.
- Is relatively inexpensive.
- May discourage through traffic.
- If used as a Wombat crossing, provides a designated pedestrian crossing place and improves pedestrian safety.
4.4 ROAD HUMPS - DISADVANTAGES
- May increase vehicle noise through braking, accelerating and vertical displacement;
- May adversely affect emergency and commercial vehicles;
- May only be used on relatively straight and flat streets away from intersections;
- Doesn't overcome 'gun barrel' effect on long straight roads.
4.5 ROAD HUMPS - MAY CONSIDER USING
- Where there is a need to reduce access road vehicle speeds as part of a LATM scheme;
- Where a speed limit of 50 km/hr. or less applies.
- Where the longitudinal gradient of a street is 10% or less.
- In Rights of Way, access roads to parks and reserves and car parks.
- As a Wombat crossing where crossing facilities are needed.
4.6 ROAD HUMPS - WHEN NOT TO USE
- On bends or crests.
- On bus routes, or where access to emergency facilities would be adversely affected, unless design is modified to provide passage of these vehicles.
- On roads with more than 4,000 Vpd, or on Distributor or higher classification roads.
- On streets with a high commercial traffic content.
Single laned slow point see drawing 200331-130
Single laned angled slowpoint see drawing 200331-131
Two laned angled slowpoint see drawing 200331-132
Two laned angled slowpoint with bicycle by-pass
Lane narrowing / kerb extensions
5.1 SLOW POINTS - GENERAL
A slow point is a device intended to reduce vehicle speeds by the creation of a short narrow section of carriageway that must be negotiated at low speed. Angled slow points can be either two lane, with or without an angled central median, or one lane. The effectiveness of an angled slow point is dependent on the degree to which the device is angled to the through roadway.Two lane devices generally only maintain deflection if they are constructed with a central median. Where road widths are sufficient, angled slow points can incorporate parking bays on the approach and departure lanes either through the use of lane narrowings and kerb extensions or marked bays. Where road widths are restrictive it may be necessary to undertake minor widening to ensure an effective deviation is created. Parallel and double offset slow points are generally single lane devices, although two lane parallel slow points are often found in association with continuous treatments incorporating parking facilities.Lane narrowings and kerb extensions are also considered to be horizontal deflection devices and are typically used to reduce speeds, improve delineation and minimise pedestrian crossing distances. In some situations kerb extensions are staggered along a street to create a meandering carriageway and embayed parking. They can be used in isolation or in combination with mid-block median treatments, roundabouts, road humps or other horizontal displacement devices.
5.2 SLOW POINTS - DESIGN CONSIDERATIONS
There is no set standard design geometry for angled or parallel slow points and this allows the flexibility to custom fit these devices to a number of unique situations.
As there are many variations possible, it is important that specific design principles are adhered to.
- The first slow point in a street should be within 100 metres of an entry statement, sharp corner or end of a street so that the approach speed may not exceed 50 km/hr.
- A minimum lane width of 3.0 metres should be maintained through the device.
- Single-lane devices are typically 5 metres long.
- Double-lane devices are typically long enough to provide a minimum island length of 10.0 metres.
- Deflection angles can vary between 10 and 30 degrees depending on the level of control required.
- Return deflection kerbs may be provided to redirect vehicles away from parked cars or cycle paths.
- Frangible bollards with reflectors may be used to delineate the shape of the device and to protect landscaping.On single-lane devices a "Give Way" sign shall be used on one approach to define priority (refer to Main Roads WA drawings200331-130 and 200331-131). Usually the sign is placed on the approach with fewer vehicles or the poorest sight distance to and beyond the LATM device.
- A D4-1-2A hazard marker may be used in lieu of the D4-1-1A hazard marker.
- The median island should be wide enough to discourage being straddled by four wheel drive vehicles and trucks. The minimum width should be 1.2m.
- To achieve desirable speed control through these types of devices it is usual to design deflection using a Standard Design Envelope ??? refer Section 12.
- Parking prohibitions may need to be applied to ensure the carriageway through angled device remains clear at all times. It may also be necessary to prohibit parking on the approach and departure lanes to ensure these are kept clear. Where the receiving carriageway width is sufficient it may be possible to provide protected parallel parking on the approach nibs of angled devices.
- Slow points potentially offer the greatest impedance to cyclists due to the squeeze points created. As it is necessary to restrict the width of carriageways through Slow Points in order to achieve the desired level of speed reduction, it is often difficult to make adequate provision for cyclists as part of a shared roadway. Special facilities such as bypass treatments or shared path sections should be provided on all cycle routes and should be considered on all other routes.
- Pedestrian movement past and across Slow Points can generally be well catered for, although landscaping can obscure sight distance for both pedestrians and drivers creating potential conflict between the two. It is important to ensure plant species chosen have appropriate growth characteristics so that sight lines over and through the landscaping is maintained.
- The impact on adjacent driveways should be carefully considered.
5.3 SLOW POINTS - ADVANTAGES
- Reduces speed in vicinity of device. Typically, single lane devices restrict speeds to about 25 km/hr. Two lane devices restrict speed to about 40 km/hr.
- When used in series, reduces speeds over the entire length of the street.
- May discourage through traffic.
- Imposes minimal inconvenience on local residents.
- When used to narrow the carriageway provides a shorter crossing distance for pedestrians.
- Relatively low cost.
5.4 SLOW POINTS - DISADVANTAGES
- Possibility of increased noise.
- Is contrary to driver expectations if used in isolation
- May restrict emergency vehicles.
- Will restrict parking opportunities.
- Can present a 'squeeze' point to cyclists if not designed and maintained correctly.
- May result in confrontations between opposing drivers arriving simultaneously at a single lane slow point
- Kerbing and signage may be damaged by vehicles travelling at inappropriate speeds.
- May not reduce motorcycle speeds.
5.5 SLOW POINTS - MAY CONSIDER USING
- Where vehicle speeds on an access road are excessive.
- Where there is substantial vehicular / pedestrian conflict.
5.6 SLOW POINTS - WHEN NOT TO USE
- On roads where sight distance is inadequate.
- On roads where there is a high number of commercial vehicles.
- Where kerbside extensions will obstruct lanes that are required for traffic.
- On bus routes.
- Where traffic volumes exceed 3,000 VPD.
6. DRIVEWAY LINKS
Driveway link - see drawing 200331-133
6.1 DRIVEWAY LINKS - GENERAL
Driveway links involve the creation of a serpentine travel path at an intersection or midblock and is typically accompanied by landscaped verges.
6.2 DRIVEWAY LINKS - DESIGN CONSIDERATIONS
Driveway links should be designed so that the following issues are considered:
- The overall device length and radii of the deflection are to be designed to suit site constraints but must be such so that motorists on either side of the device have the ability to see from one side to the other without obstruction.
- Lane widths of 3.0 metres and less or 3.7 metres and greater are desirable. Consideration should be given to the manoeuvring requirements of all vehicles likely to be using the link including garbage trucks, emergency and other commercial vehicles.
- The needs of cyclists and pedestrians should be considered and where warrants exist, consideration needs to be given to providing a separate cycle and / or pedestrian facility through the device.
- Where the device is landscaped, consideration must be given to ensuring sight distances are maintained.
- Bollards with retro-reflective markers may be used in lieu of unidirectional RRPM's at the entry of the device. Where bollards are used, care is to be taken to ensure they do not obstruct hazard markers (refer to Main Roads drawing 200331-133).
- A Give Way" sign shall be used on one approach to define priority (refer to Main Roads drawing 200331-133) Usually the sign is placed on the approach with fewer vehicles, or the approach with the poorer sight distance to and beyond the LATM device.
6.3 DRIVEWAY LINKS - ADVANTAGES
- Reduces speed in vicinity of device.
- May discourage through traffic and heavy traffic.
- Can be extensively landscaped to reduce the "gun-barrel" effect of streets.
- Imposes minimal inconvenience on local residents.
- May be used to impose one-way flow for use only for certain classes of vehicle eg, buses and bicycles.
6.4 DRIVEWAY LINKS - DISADVANTAGES
- Possibility of increased noise.
- May be contrary to driver expectations if used in isolation.
- May restrict emergency vehicles.
- Will restrict parking opportunities.
- Can present a 'squeeze' point to cyclists if not designed and maintained correctly.
- Confrontations between opposing drivers arriving simultaneously at a single lane slow point.
6.5 DRIVEWAY LINKS - MAY CONSIDER USING
- Where vehicle speeds on an access road are excessive;
- Where traffic volumes are low.
6.6 DRIVEWAY LINKS - WHEN NOT TO USE
- On roads where sight distance is inadequate;
- On roads where there is a high number of commercial vehicles;
- On bus routes;
- Where traffic volumes exceed 1,000 VPD
7. MODIFIED "T" INTERSECTIONS
Modified "T" intersection with normal priority
Modified "T" intersection with reassigned priority (see drawing200331-134)
7.1 MODIFIED "T" INTERSECTIONS - GENERAL
Modified 'T' intersections rely on constructing a kerbed blister at the head of a T-intersection and where possible central curved medians to affect a change in a vehicle's travel path. As such they act in a similar manner to slow points in moderating traffic speeds through the head of a T-intersection. When used in series they can provide speed control down the length of a street. Modified 'T' intersections can also be used to change the priority of T-intersections by treating two ninety-degree legs as the priority carriageway. It is common to treat the lesser leg of the intersection with an entry statement to reinforce the status of the through carriageway.
7.2 MODIFIED "T" INTERSECTIONS - DESIGN CONSIDERATIONS
'T' intersection modifications should adhere to the design principles governing channelisation design. Important considerations are:
- Lane widths should allow service vehicles to negotiate the intersection.
- Sight distance.
- Geometric elements should be designed on the basis of achieving deflections similar to those used in roundabout design.
- If 'T' intersections are used to change priority, consideration should be given to the need for the installation of "Give Way" or "Stop" signs.
- Splitter islands, where used, should have semi mountable kerbs.
- Drainage considerations may limit the geometry of the device.
- Carriageway widths adjacent to islands should be wide enough to allow a cyclist and vehicle to pass side by side or bypass facilities should be provided.
- Where landscaping is provided this should not obscure sight distances.
- Wherever possible, medians should be placed to minimise impact on property access.
- Unbroken separation lines or painted medians should be used on all approaches to splitter islands. and raised medians.
7.3 MODIFIED "T" INTERSECTIONS - ADVANTAGES
- Creates orderly movement at intersection.
- Reduces speed in the vicinity of the device.
- When placed in series can lower vehicle speeds along the length of the street.
7.4 MODIFIED "T" INTERSECTIONS - DISADVANTAGES
- May impact on property access.
- Can create a squeeze point for cyclists.
- Can reduce on street parking spaces.
- May reduce sight distance.
- May be restrictive for buses and emergency and service vehicles.
7.5 MODIFIED "T" INTERSECTIONS - MAY CONSIDER USINGWhere there is a need to regulate traffic movements.
- Where there is a need to moderate speeds without displacing traffic.
- Where crash numbers are high.
- To change the bias between through and turning movements.
- To accommodate an existing bias between through and terminating legs.
7.6 MODIFIED "T" INTERSECTIONS - WHEN NOT TO USE
- On crests where sight distance is limited.
- On streets where carriageway widths are less than 7 metres.
- On distributor class roads or higher.
7.7 MODIFIED "T" INTERSECTIONS - Change of Stop or Give Way Priority at Four-Way and 'T' IntersectionsGuidelines Change of Stop or Give Way Priority at Four Way and T Intersection Document
8. BLISTER ISLANDS
Blister island see drawing 200331-135
Centre blister with on-road bicycle by-pass
8.1 BLISTER ISLANDS - GENERAL
Blister islands are elliptically shaped islands constructed mid block to restrict traffic speeds. They are commonly used in situations where the tight configuration of angled slow points is not acceptable. This situation commonly occurs on bus routes and higher classification roads where provision needs to be made for commercial traffic.
8.2 BLISTER ISLANDS - DESIGN CONSIDERATIONS
Blister island geometry can be modified to suit specific circumstances; however the following basic design principles should apply:
- Island width can vary but should not be less than 2 metres.
- Island length can vary but should not be less than 10 metres.
- Lane widths should be 3 metres or less or 3.7 metres or greater. Typically, lane widths past the island are 4.5 metres on the approach and 5.5 metres on the departure.
- Where approaches to the blister island incorporate "nibs" to narrow the approach, hazard markers (D4-1-2A) should be erected on the nibs.
- Deflection through the device should be designed according to the same principles applied to roundabout design so as to achieve required speed reductions.
- Where landscaping is provided this should not obscure sight distances, including pedestrian sight distance for crossings located on blister islands. Needs of small children and people with disabilities should always be considered.
- Blister islands should be located to minimise impact on property access.
- Kerbing should have a semi mountable profile.
- Where pedestrian crossings are incorporated in blister islands, appropriate geometry and pedestrian grab rails are provided.
- The use of W5-33 (slow point) warning signs with an appropriate advisory speed limit is mandatory.
- Unbroken separation lines should extend 4.5 metres past the approach ends of the central islands.
- Delineation must clearly define the desired travel paths.
- Tracking movements through the device should take into account the requirements of buses and in particular low profile buses where these are being utilised on the route.
- On all routes, consideration should be given to provide for a bicycle by-pass, either on or off-road. On Perth Bicycle Network routes, a bicycle by-pass should be provided.
8.3 BLISTER ISLANDS - ADVANTAGES
- Well-spaced blister islands reduce speeds along a street.
- They can provide refuge for cyclists and pedestrians crossing the street.
- When landscaped they can reduce a 'gun barrel' effect on long straight roads and enhance the residential streetscape.
- Can accommodate buses and commercial traffic.
- Design flexibility allows for a variety of applications over a range of roads having different traffic volumes.
8.4 BLISTER ISLANDS - DISADVANTAGES
- Reduction of on street parking adjacent to the islands.
- May create a squeeze point for cyclists.
- May impact on property access and egress.
8.5 BLISTER ISLANDS - MAY CONSIDER USING
- Where vehicle speeds on a road are excessive.
- Where there is a need to break long straight lines of sight.
- On Local Distributors and Access Roads where the posted speed limit is under 50km/h.
8.6 BLISTER ISLANDS - WHEN NOT TO USE
- On narrow carriageways where substantial islands cannot be fitted.
- On District Distributor roads where the geometry will likely result in a transference of traffic to adjacent routes.
- On Primary Distributor roads.
9. PARTIAL CLOSURES
Partial closure restricting access to "left turn entry only" off the main road -
"Seagull" closure restricting movements to "left in, left out" only -
10. FULL CLOSURES
Full closure - see drawing 200331-138
10.1 FULL CLOSURES - GENERAL
Full road closures provide practitioners with the opportunity to regulate the movement of traffic through streets by way of mid block closures, or through intersections by the closure of one or more legs of the intersection. In this manner they can be successfully used to deter 'bypassing' traffic from local streets or to reduce conflict points on intersections thereby reducing crash frequencies.
10.2 FULL CLOSURES - DESIGN CONSIDERATIONS
When designing a full closure the following principles should be followed:
- The location of road closures should be carefully selected so that unacceptable volumes of traffic are not redirected to unsuitable routes.
- Turning circles should be provided, either as shown on the drawing or in another acceptable format to facilitate all anticipated turning movements.
- Closures should not be placed over a crest or in other situations where insufficient stopping sight distance is available.
- "No Through Road" signs should be installed at the last entry to the closed section of the road.
- Wherever possible cycle access from cul-de-sac heads should be provided. Where this provision is made, "Bicycles Excepted" drop tags should be installed.
- Parking prohibitions should be introduced to ensure turning areas are unobstructed and vehicles can complete turning movements.
- Full closures should include provision for breaks or diversion paths that allow pedestrians and cyclists to access adjoining roadways or pathways. These facilities should be designed in such a manner so that adequate protection in the form of bollards or similar deterrents is provided to prevent violation by motor vehicles.
10.3 FULL CLOSURES - ADVANTAGES
- Eliminates non local traffic.
- Reduces intersection crashes by eliminating conflict points associated with the closed leg.
- Increases pedestrian safety.
- Can create good cycle access.
- Can be landscaped.
10.4 FULL CLOSURES - DISADVANTAGES
- May increase traffic volumes on adjacent routes.
- Reduced accessibility for local residents.
- Reduced accessibility for service and emergency vehicles.
- Increased trip lengths and travel times for some road users.
- May reduce the availability of on road parking.
10.5 FULL CLOSURES - MAY CONSIDER USING
- As part of a LATM scheme to discourage traffic bypassing busy distributor roads and using local streets as 'rat runs'.
- To eliminate right turning traffic from busy distributor roads where right turn pockets are not available and turning traffic impacts on the following through traffic.
- At intersections where crash history indicates a high number of right angle and right turn through crashes.
- At intersections where sight distances are substandard and turning movements are potentially dangerous.
10.6 FULL CLOSURES - WHEN NOT TO USEWhere unacceptable levels of traffic transference into adjacent streets is expected.
Combined painted and raised median see drawing 200331-0139
Median treatment see drawings 200331-0140 & 200331-141
Installation of median treatments leading to cyclist squeeze points should be avoided
11.1 MEDIAN TREATMENTS - GENERAL
11.2 MEDIAN TREATMENTS - DESIGN CONSIDERATIONS
Median islands are designed in accordance with the general principles governing channelisation design. Important considerations are:
- When placed at intersections median islands should provide for turning movements of all traffic commonly using the intersection.
- Austroads Guide to Traffic Engineering Practice Part 10 - Local Area Traffic Management states the following with respect to the design of lane widths:
"Care needs to be taken that the introduction of LATM treatments that narrow the road carriageway width do not create safety problems for cyclists.
Practice should be that lane widths are either designed to be wide enough in all instances to allow the safe passage of a cyclist and a vehicle side by side (3.7m or more) or narrow enough to permit the passage of a vehicle or bicycle only (3.0m or less). Widths in between these two extremes create squeeze points and result in conflicts."
Bearing this in mind, carriageway widths adjacent to median islands shall consider the needs of cyclists and, where warranted, be wide enough to allow a cyclist and vehicle to pass side by side. If the route is a bus route, additional width may be required. The following minimum widths should be provided:
|Local road (not a bus or cycle route and where significant numbers of child and/or inexperienced cyclists are unlikely to occur)||
|Bus route but not a cycle route||= 3.7m.|
|Cycle route but not a bus route||= 3.7m.|
|Bus and cycle route||= 4.2 m.|
- If parallel parking is to be permitted along a section of road, the carriageway widths should be wide enough to accommodate a parked car as well as the mixture of traffic. In all cases an additional 2.5m width should be added to the minimum lane width given in the previous dot point to accommodate parking.
- Median island widths should be as follows:To accommodate signals, signs or street lighting; minimum = 1.5m, desirable minimum = 2.0m.
- To accommodate pedestrians and gophers; minimum = 2.0m, desirable minimum = 2.5m.
- To accommodate cyclists; minimum = 2.0m, desirable minimum = 3.0m.
- Where landscaping is provided this should not obscure sight distances either along the road or when exiting abutting properties.
- Wherever possible medians should be placed to minimise impact on property access.
- Pedestrian crossings should be co-linear, and where fitted TGSI's should align with the direction of travel.
- On painted medians chevron markings are mandatory where the road and median surfacing are the same colour. Painted medians should not extend beyond 200m without the introduction of a raised median island.
- Frangible trees (<100mm diameter) may be used in kerbed and painted medians to reduce traffic speeds.
11.3 MEDIAN TREATMENTS - ADVANTAGES
- Provides refuge for pedestrians.
- Separates opposing traffic.
- Enforces lane discipline on curves.
- Enforces no right turns, when placed across an intersection on the through road.
- Well accepted by residents.
- Can restrict speed.
- Painted medians can provide storage for turning vehicles at driveways.
- Accommodates centrally displayed traffic control devices.
11.4 MEDIAN TREATMENTS - DISADVANTAGES
- May impact on property access.
- Can create a squeeze point for cyclists.
- May impact on kerbside parking.
11.5 MEDIAN TREATMENTS - MAY CONSIDER USING
- Where there is a need to regulate traffic movements.
- Where there is a need to provide staged refuge for pedestrians.
- Where there is a need to moderate speeds without displacing traffic.
- On distributor or higher classification roads.
11.6 MEDIAN TREATMENTS - WHEN NOT TO USE
- On narrow carriageways.
- Where there is insufficient sight distance to provide a safe crossing point for pedestrians, because pedestrians will assume they can cross safely
Standard design envelopes are used to design the horizontal displacement necessary through devices such as roundabouts, blister islands, modified "T" junctions and slow points to reduce vehicle speeds to a desirable level.
The tighter the horizontal curvature at a roundabout or other form of slow point, the slower will be the speed at which drivers traverse the device. The design of stand-alone devices is governed by the initial choice of desirable 85th percentile speed through the device. Once this is known, geometry that restricts the virtual radius through the device can be prescribed so as to achieve the desired speed.
Graphs and tables from the American Association of State Highway and Transportation Officials' (AASHTO) "A Policy on Geometric Design of Highways and Streets" relate horizontal curvature to operating speed based on the formula:
R is the horizontal curve radius (m),
V is the speed of travel around a curve (km/h),
e is the superelevation rate, and
f is the side-friction factor.
Applying this formula to a range of speeds and assuming side friction factors recommended by AASHTO and a range of superelevation values, gives the following result.
Drawing number 200331-143 provides guidance on the design of angled slow points using the Standard Design Envelope.
|The following Main Roads drawings are applicable to this guideline: