Why introduce traffic calming?
The concept of traffic calming is primarily concerned with reducing the adverse impact of motor vehicles on built up areas. This usually involves reducing vehicle speeds, providing more space for pedestrians and cyclists, and improving the local environmental quality. Traffic control devices such as speed limit signs are regulatory measures that require enforcement. By contrast, traffic calming measures are intended to be self-enforcing. Traffic calming measures may have many objectives (Traffic Calming for Communities):

• achieving slow speeds for motor vehicles;
  
• reducing collision frequency and severity;
  
• increasing the safety and the perception of safety for non-motorized users of the streets;
  
• reducing the need for police enforcement;
  
• enhancing the street environment (e.g., street scaping);
  
• increasing access for all modes of transportation and
  
• reducing through motor vehicle traffic.

Traffic calming can be a way of resolving potential conflicts and competition for road-space but it has to be developed in an integrated way. Some specific policy issues may be involved (IHT, 1997):

• Buses
  Specific attention needs to be paid to the design of traffic calming measures on bus routes, because, for example, road humps appear to be a problem for bus passenger comfort and safety and for vehicle maintenance.
  
• Vulnerable road users
  Traffic calming measures, which distract the attention of vulnerable road users such as children, the elderly and mobility-impaired people from their purpose of crossing or walking along the road, may become a hazard which causes accidents rather than reduces them.
  
• Emergency services
  It is important to maintain good access and a rapid response time for emergency services.
  
• Routeing of heavy goods vehicles
  Heavy goods vehicles should be encouraged to remain on the highest available category of route for as much of their journey as possible. Traffic calming can be used to control speeds but the largest size of vehicles involved needs to be taken into account.
  
• On-street parking
  Provision for the required levels of on-street parking should form an integral part of the design of all traffic calming and parked vehicles themselves can sometimes assist in reducing traffic speed, if they are sited in appropriate locations.
  
• Economic development
  Traffic calmed areas have potential for economic growth and development and this can be a positive encouragement to shopping and other commercial activity, as the progressive pedestrianisation of town centres has demonstrated.
  
• Main roads
  Successful traffic calming requires a road hierarchy framework. Traffic calming techniques may be appropriate where drivers need to be encouraged to proceed at a pre-defined speed in a calm and safe manner.
  
• Safe routes to school
  Community representatives and school authorities should be consulted to ensure that traffic calming proposals can assist children on their journey to school.

Demand impacts
Traffic calming measures reduce vehicle traffic speeds and volumes, so that the main impacts of these measures can be to improve the environment and to reduce accidents. The purpose of segregation measures is to discourage or eliminate through traffic, but to induce diversion to other roads. The additional distance travelled is likely to add only marginally to the cost of the journey, however, and hence to have little impact on the number of journeys by car. Only where the network is close to capacity is demand likely to be reduced.

Responses and situations
Response	Reduction in road traffic	Expected in situations
		N/A
		Where the drivers can divert from the calmed streets or roads.
		Where traffic calmed areas can be less attractive and so discourage shopping; but can have potential for economic growth and development by pedestrianisation.
		Where there is potential to shop from home.
		Where improving conditions for cyclists and pedestrians.
		N/A
		Reduce or increase where some people may move house into the calmed area to improve safety and local environment.

Short and long run demand responses
It is unlikely that there will be significant change in demand response of traffic calming over time.

Supply impacts
The main impact on supply of traffic calming is to reduce the capacity of the road network. The scale of this will be greatest where traffic calming measures are applied to main roads. Reductions in capacity are also likely to be sizeable where segregation measures, using mazes or traffic cells, are implemented. In these cases, the connectivity of the minor road network is reduced, and through traffic and some local traffic is forced to use the main roads. The impact will depend on the extent of the measures, but it is possible to envisage reductions in capacity of as much as 10%. With integration measures, the impact on capacity will be much less, since the minor roads are still available routes when demand is at the highest.

Financing requirements
The cost of traffic calming varies according to the measures and countries. Where considerable environmental measures are used to complement the physical measures the cost rises significantly.

The Dutch "Woonerf" required the reconstruction of the street and the removal of kerbs and footways to achieve a common shared space and were therefore very expensive, typically over £25 per square metre of road in the mid 1980s, which has constrained their widespread use. An indication of traffic calming costs, based on mid-1980s prices, from a selection of schemes in the Netherlands and Germany, range from under £1 per square metre of street area to over £100. The "standard" traffic calming techniques such as plateau, gateways, junction treatments and planting fall into the £5-£20 per square metre range (Harvey, 1992).

In Britain Kent County Council quote prices of £700-1,000 (in 1991?) for a single road hump, including signing and marking, but not lighting. Block paved ramped narrows are quoted at £3,000 for single way and £5,000 for two-way. Recent estimates by Leeds City Council for plateau range from £6,000-£15,000 per measure, with the former requiring no road narrowing or re-kerbing and being constructed in standard bitumen, while the more expensive measure would include for re-paving and re-kerbing to narrow the road, and construction in block paving or asphalt (Harvey, 1992).

The following table provides sample cost estimates for various traffic calming measures. These estimates cannot replace detailed cost estimates using quantities and local unit prices for work items associated with specific projects. The estimates in this table may be useful in conceptual planning, as they show order of magnitude differences among measures. Costs increase quickly when measures require landscaping, drainage improvements, or land acquisition (ITE and FHWA, 1999).

(ITE and FHWA, 1999)

Expected impact on key policy objectives
The immediate purpose of traffic calming is to reduce the speed and volume of traffic. Reductions in traffic speed and volume are just means to other ends such as traffic safety and active street life, livability and the local environment, but can also induce re-routing.

Expected impact on problems
As with impacts on objectives traffic calming measures has potential to contribute to the alleviation of a number of key problems through reduction of the speed and volume of traffic, but the scale of contribution is dependent on the individual measures.

Expected winners and losers

Barriers to implementation