7.3 Horizontal Deflection Devices

Horizontal deflection devices are designed to change the horizontal course or path of a vehicle as the result of a physical feature of a roadway. This deflection generally discourages short-cutting or through traffic to a varying extent and may achieve a significant reduction in traffic volume, speed and conflicts.

Horizontal deflection devices should be clearly visible to approaching drivers, illuminated by adequate street lighting and enhanced by the use of signs and other linemarking if necessary. The manoeuvring of large vehicles should be determined by using relevant turning templates. Consideration needs to be given to maintaining drainage paths and where possible, providing bypasses for bicycles.

7.3.1 Lane Narrowings/Kerb Extensions

Description of lane narrowings/kerb extensions

Lane narrowings involve the narrowing of the trafficable carriageway to reduce speeds, improve delineation and to minimise pedestrian crossing distances (and therefore exposure to conflict). It is generally done by extending the kerbs inwards or via other forms of kerb modifications but it can also be achieved through the introduction of on-street parking. When designing these devices, careful consideration should be given to the need for bicycles to pass clear of the extension either adjacent to the traffic lane or via other means, taking into account the likely risks to cyclists, the demand for cycling at the treatment location, and issues relating to site constraints. Kerb extensions should be clearly visible by approaching drivers, illuminated by adequate street lighting and enhanced by the use of signs and road marking. Careful consideration should be given to maintaining drainage paths without creating a potential hazard to cyclists and pedestrians.

Application of lane narrowings/kerb extensions

It is appropriate to use lane narrowings/kerb extensions in: • commercial areas • low-speed residential environments

It is inappropriate to use lane narrowings/kerb extensions: • Where the kerbside lane is required for traffic • In locations with limited sight distance • In streets without adequate street lighting

• where the narrowing is such that it will pose a difficulty to buses and cyclists on fixed routes.

The effectiveness of lane narrowings/kerb extensions can be increased when used in combination with: • Median treatments including splitter islands • Flat-top road humps/wombat crossings/raised pavements • Road humps/cushions • Roundabouts.

Advantages of lane narrowings/kerb extensions

The advantages of lane narrowings/kerb extensions include: • A shorter crossing distance for pedestrians • They may improve the visibility of pedestrians and vehicles • A reduction in vehicle speeds, particularly on curvilinear alignments • Relatively low cost • To delineate and protect parking spaces • Providing an opportunity for landscaping • They have relatively little effect on emergency vehicles • Significantly less disruptive to local traffic than some other forms of latm devices that are more severe

in their design.

Disadvantages of lane narrowings/kerb extensions

The disadvantages of lane narrowings/kerb extensions include: • They may reduce the amount of available kerbside parking • Bicycle lanes may be difficult to accommodate

Appendix "E"

• Drivers may mistake an empty kerbside parking lane for a traffic lane • They may introduce squeeze points and increase the conflict between motor vehicles and cyclists • They are less effective than many other horizontal displacement devices in reducing speeds • Parking manoeuvres may be difficult on heavily trafficked streets • They may increase congestion.

Examples of lane narrowings/kerb extensions

Examples of lane narrowings/kerb extensions are illustrated in Figure 7.10.

Figure 7.10: Examples of lane narrowings/kerb extensions

City of Yarra, Victoria Town of East Fremantle, Western Australia

City of Glenorchy, Tasmania City of Mitcham, South Australia

7.3.2 Slow Points

Description of slow points A slow point is a series of kerb extensions on alternating or opposite sides of a roadway, which narrow and/or angle the roadway. Slow points are intended to reduce vehicle speeds. Slow points can be either one or two lanes wide and can be angled. In a two-lane slow point, a median island is generally very effective in separating opposing traffic. This will also provide a greater visual restriction and it can be used as a pedestrian refuge if designed appropriately.

Application of slow points

It is appropriate to use slow points on local streets where: • Vehicle speeds are considered excessive • There is a high proportion of through traffic • The resulting traffic volume will be low (not more than 1000 vehicles per day) otherwise congestion

and crash risk may increase.

Appendix "E"

It is inappropriate to use slow points: • On bus routes • At locations where the resulting sight distance to the device will be inadequate • On streets with a high connective role in the local street network • On streets where on-street parking is in short supply and its removal will significantly impact on

adjacent properties (e.g. Where they do not have access to off-street parking) • Routes leading to emergency facilities, e.g. A hospital • Streets where there is a high number of commercial vehicles (unless the aim is to divert this type of traffic).

When designing slow points the following should be considered: • Design for a maximum speed through the device of 10–20 km/h • A lane width between 2.8 and 3.0 m should be maintained through the device • Deflection angles may be varied in the range of 10° to 30° depending on the level of control required • Raised kerb returns should be provided to redirect vehicles away from parked cars, pedestrian

paths, bicycle bypasses, and adjacent properties • On-street parking should be considered in the design to ensure the device remains clear at all times at

the entry and exit of the device • Adjacent driveways should be taken into account • An appropriately designed bicycle bypass may be provided, based on an assessment of relative risk

and demand for cycling, so long as it does not compromise the speed reduction benefits of the design • The device should be lit and signed to the appropriate standard.

The effectiveness of slow points can be increased when used in combination with lane narrowings, median treatments, centre blister islands and threshold treatments.

Austroads (2009b) suggests slow points produce an 85th percentile speed reduction of up to 34% at the treatment.

Advantages of slow points

The advantages of slow points include: • A reduction in vehicle speeds in the vicinity of the device and when used in a series, speeds are

reduced over the length of the street • A significant reduction in road crashes • They may provide pedestrians with a shorter distance to cross the street • They discourage through traffic • They impose minimal inconvenience on local residents • They can provide a landscaping opportunity.

Disadvantages of slow points

The disadvantages of slow points include: • They may restrict emergency vehicles and buses • Possible increase in traffic noise • They will require the removal of on-street parking • With one-lane devices, confrontations between opposing drivers may occur when arriving

simultaneously and it may be unclear who should give way • They can be hazardous for cyclists if they are not catered for in the design • Landscaping needs to be maintained so as not to reduce visibility.

Two-lane slow points are usually less effective than one-lane slow points in controlling speeds and providing an adequate visual obstruction.

Appendix "E"

Examples of slow points

Examples of one-lane slow points are illustrated in Figure 7.11 and two-lane slow points in Figure 7.12. A diagrammatic illustration of the two types of angled slow point is provided in Figure 7.13.

Figure 7.11: Examples of one-lane slow points

City of Prospect, South Australia City of Christchurch, New Zealand

City of South Perth, Western Australia City of Prospect, South Australia

Figure 7.12: Examples of two-lane slow points

City of South Perth, Western Australia City of Stirling, Western Australia

Appendix "E"

Figure 7.13: Two main types of angled slow point

Appendix "E"

7.3.4 Driveway Links

Description of driveway links

Driveway links take the form of a single-lane two-way meandering road extending over the length of two or more property frontages. They are an extended form of a slow point that generally provides a greater visual and physical impact on the street and the amount of traffic using it. Passing points may be required along the link if it is either very long or it is curved such that approaching drivers cannot see to the far end. Driveway links are particularly effective in reducing through traffic. Consideration needs to be given to maintaining drainage paths and providing bypasses for bicycles where possible.

Driveway links often incorporate extensive landscaping and care needs to be taken that sufficient sight distance is retained. Paving materials should contrast with the adjacent street surface.

Application of driveway links

It is appropriate to use driveway links where:

• There is a high proportion of through traffic

• Full or partial road closures are not appropriate

• Vehicle speeds on a street are less than 50 km/h

• The resulting traffic volume will be low (not more than 1000 vehicles per day) otherwise congestion and crash risk may increase

• There is a need to break long, straight lines of sight.

It is inappropriate to use driveway links on:

• Bus routes

• Streets with a high connective role in the local street network

• Streets where on-street parking is in short supply, it cannot be replaced in the design, and its removal will significantly impact on adjacent properties (e.g. Where they do not have access to off-street parking)

• Where access to properties by service vehicles will be prevented

• Routes leading to emergency facilities, e.g. A hospital.

Driveway links are an effective treatment if installed in isolation but can also be quite successful if implemented in series. Two or more driveway links should not be installed in the same section of a street (i.e. between intersections) as this may prevent access to properties by service vehicles.

Advantages of driveway links

The advantages of driveway links include:

• A reduction in vehicle speeds

• Discouragement of through traffic

• An increase in pedestrian safety

• The provision of greater visual and physical impact than slow points

• They visually enhance the street through landscaping and reduce the ‘gun barrel’ effect on long straight roads.

Appendix "E"

Disadvantages of driveway links

The disadvantages of driveway links include:

• They may restrict emergency vehicles and commercial vehicles and are not suitable for buses

• They will reduce the amount of on-street parking

• They can be hazardous for cyclists if they are not catered for in the design

• Confrontations between opposing drivers may occur and it may be unclear who should give way

• Landscaping needs to be maintained so as not to reduce visibility

• They are an expensive device.

Examples of driveway links

Examples of driveway links are shown in Figure 7.16 and Figure 7.17 illustrates a typical layout.

Figure 7.16: Examples of driveway links

City of Prospect, South Australia City of Stirling, Western Australia

City of Port Adelaide, South Australia City of Subiaco, Western Australia

Appendix "E"

Figure 7.17: A typical driveway link treatment

Appendix "E"