The following information on Traffic Calming is extracted from a report prepared by Council Officers for Council’s City Services Committee in March 2008. It provides some background information on traffic calming, with the emphasis on engineering mid-block treatments (physical treatments on sections of road between intersections) to reduce vehicle speed.
Traffic Calming is the term given to measures utilised to attempt to improve the safety or amenity of roads, by either reducing the volume of traffic or reducing the speed of traffic using that road.
Generally traffic calming measures would have an emphasis on residential streets, or streets with high proportions of vulnerable road users (such as school children or elderly pedestrians).
There are typically three categories of traffic calming measures:
Enforcement is the responsibility of the Tasmanian Police.
Education typically occurs on a statewide basis, and is driven by the Department of Infrastructure, Energy & Resources (DIER).
Engineering treatments on local roads are generally initiated by Council and are the category that will form the focus of this report.
Engineering treatments can be broken down into two main categories, namely intersection treatments and mid-block treatments.
Traffic calming at intersections generally takes the form of devices installed to increase the safety of an intersection. There are numerous examples, including the installation of roundabouts, traffic signals, traffic islands. the banning of turning movements etc.
In relation to the restricting of vehicle speeds, the most effective treatment is typically the installation of a roundabout, due to drivers on every approach to a roundabout being required to slow down to a speed sufficient to allow them to give way to other traffic.
The installation of a roundabout will typically result in a significant increase in the safety of an intersection, as well as a reduction in overall vehicle speeds.
There are three classes of device that could be used in a mid-block location to reduce vehicle speeds. They are vertical displacement devices (such as road humps), horizontal displacement devices (such as chicanes) or a road narrowing treatment (such as kerb outstands or the installation of median treatments).
Vertical displacement devices rely on discomfort to the driver caused by the vehicle rapidly going up then down over an object forcing the driver to reduce their speed.
A horizontal displacement device slows vehicles by forcing them to perform a difficult driving task and moving their vehicle from side to side whilst negotiating a system of kerb bulbings and traffic islands. These devices tend to work well on residential streets that do not have to cater for large trucks /buses. The need for devices to be negotiable by large vehicles including garbage trucks would require the horizontal displacement device to be constructed with more generous proportions, reducing the effectiveness of the device on standard cars.
A treatment to narrow the width of the road available to through traffic can also potentially reduce traffic speed. This can be achieved by the allowing of on-street parking, the planting of trees, the use of linemarking and the installation of median treatments.
All the above mid-block treatments have certain advantages and disadvantages.
Generally traffic calming measures in mid-block locations would have an emphasis on the reduction of vehicle speeds. Note that all devices are designed and installed to the approval of the Department of Infrastructure, Energy & Resources.
In Australia, Speed/Road Humps have been a common treatment for roads with unacceptably high traffic speeds since the 1980’s.
Currently in Tasmania, three types of Road Humps have been installed on the road network:
Watts Profile Road Hump Installation
Flat Top Hump Installation
Speed Cushion Installation
In the 1980’s when road humps were first introduced, numerous investigations were undertaken into the effectiveness of humps in reducing vehicle speeds. The following results were obtained for “Watts Profile” and “Flat Top” humps (Speed Cushions are a more recent innovation). It should be noted that the studies were undertaken prior to the implementation of the 50 km/h speed limit on residential streets and as such the 85th percentile (85%ile) traffic speed prior to the installation of Speed Humps would have been about 60 km/h. The results quoted below are obtained from ‘Towards Traffic Calming: A Practitioners Manual of Implemented Local Area Traffic Management and Blackspot Devices’ – Federal Office Of Road Safety - 1993:
Narrower speed cushions are available, with 1.6m wide units installed by the City of Glenorchy in Highfield Street being found to reduce the 85%ile vehicle speed from 60 km/h to 50 km/h midway between installations.
Chicanes can be installed to force drivers to perform a ‘shimmy’ manoeuvre between obstacles, which generally results in slower vehicle speeds.
Treatments such as these are constrained somewhat by the need for them to be negotiable by large vehicles such as garbage trucks, which results in them being able to be driven through quickly by confident car drivers. There is also anecdotal evidence that they are attractive to ‘hoons’ in that driving through one at speed presents a challenge of sorts.
Treatments of this type can also be difficult to install in locations where on-street parking is in high demand and there are a large number of residential driveways (which such devices must be installed between so as to not block driveway access). The ongoing maintenance costs can also be higher if landscaping is to be properly maintained.
Speed reductions can also be achieved by reducing the width of the road available to drivers.
A common treatment on wide roads used in Tasmania is the installation of median islands and ‘S2’ linemarking to form a median down the centre of a road reserve. This is generally installed to provide a safe, sheltered turning area for vehicles turning into side roads and driveways, and also to make it safer and easier for pedestrians to cross the street.
By reducing the apparent width of the road that is available to drivers, a reduction of vehicle speed of between 2 km/h and 5 km/h could typically be expected.
Median Treatment – Arterial Road
Median Treatment – Residential Street
A median treatment such as those shown above can only be installed on roads where the width between kerblines is 12 metres or greater.
Another means of reducing the apparent width of road available to traffic is to install continuous edge linemarking, as shown in the figure below.
Continuous edge linemarking such as this can be effective in achieving a speed reduction of in the order of 2 km/h to 5 km/h. It is generally most effective on sections of roads where there is little or no on-street parking (as the presence of parked cars generally provides lateral restraint and eliminates the benefit of edge linemarking).
Continuous Edge Linemarking Treatment – Collector Road
Another effective means of achieving a reduction in vehicle speeds in the 2 km/h to 5 km/h range on arterial routes is the introduction of marked bicycle lanes. These effectively reduce the clear width available to traffic, and also have the benefit of providing a safer environment for cyclists.
On residential streets, narrow pavement widths such that drivers travelling in opposing directions must be cautious when passing each other can be effective in keeping speeds low.
The figure below shows a narrow residential street which carries quite a high volume of through traffic (Andrew Street in North Hobart). The presence of parking on both sides means there is not sufficient width for two cars to pass one another. Drivers passing one another are required to give way. This works to both reduce traffic speeds and make the route less attractive as a through traffic route. It can create conflict between drivers, if drivers do not display courtesy.
Narrow Road Width – Collector Road
Another means of traffic calming in mid-block locations is the installation of kerb outstands and median islands periodically along a section of road, to reduce the trafficable width. Whilst this does not physically slow a motorist, some small reduction in speed can be achieved by these devices which send a message to motorists that the street is a residential street where a slower speed is appropriate.
This type of treatment can produce good results from a landscaping and residential amenity perspective, however, it can be relatively expensive for very little gain in speed reduction.
Aside from the treatments described above, traffic calming can also be achieved by more severe methods. This could include the closure of access to a residential street from one end (effectively turning the street into a cul-de-sac), or the turning of a road into a one-way street, or other similar treatments to reduce the amount of through traffic.
Whilst these types of treatments can be very effective, they also can be very divisive in that they may transfer problems to other nearby streets and typically require strong justification to implement.
Another treatment type that has been shown to be effective overseas is the ‘naked’ streets approach. At its most basic level, it involves the removal of traffic control devices such as ‘Stop’ signs, ‘Give Way’ signs, holding lines, traffic signals etc. This is based on the theory that removing road users’ certainty will lead them to be more aware and drive at slower speeds, as they must be constantly on the lookout for what other road users are doing. This approach has not been attempted in Hobart, but may be appropriate in some areas where pedestrian volumes and vehicular traffic are moderately high. A decision on such an approach would need State Government endorsement. The cost of streetscape work to safely integrate the ‘free’ movement of pedestrians and vehicles can be high and only justified in inner city or high pedestrian flow precincts.
Traffic calming measures have the potential to provoke strong feelings in the community.
In particular, treatments that restrict driver access (such as banning turning movements, making streets one-way etc), or present an obstacle to drivers (such as road humps) can be a source of controversy.
One of the most effective and least controversial methods of traffic calming is the installation of roundabouts at intersections.
In mid-block locations, the most widely accepted calming measure is the installation of median treatments, and other treatments that reduce the available trafficable width but such measures are not as effective nor indeed often possible in already narrow residential streets.
Council has a limited amount of funds available for capital and maintenance works and in terms of cost effectiveness, speed humps are the leading treatment of the speed reduction measures. The speed cushion using recycled rubber is by far the most effective treatment.
Traffic calming is undertaken to modify driver behaviour such that the behaviour is considered reasonable for the environment in which they are travelling.
Education and Enforcement are two powerful tools for modifying driver behaviour. Engineering is the third tool, the physical modification of a road environment to force drivers to change behaviour.
Engineering solutions are typically high cost, which are generally only appropriate in specific locations where there is a demonstrated problem. Only after careful consideration of quantitative data (speeds, volumes, crash rates, user groups) and qualitative (resident and other stakeholder views) does Council implement traffic calming.
Based on many years experience, Council Officers are of the opinion that where the installation of mid block treatment for speed reduction is required, it comes down to a choice between flat top humps of the type installed in Lipscombe Avenue or the recycled rubber speed cushions especially where there are regular movements of large trucks or buses. In making such a choice it is also relevant that the flat top hump is 2 to 3 times more expensive than the speed cushions ($20,000 vs $8,000).
For intersections, the best treatment is generally very specific to the site. In terms of acheiving reductions in speed, and improvements in safety, roundabouts are generally recognised as the best treatments.
