First principles assessment
Why introduce traffic calming?
Demand impacts
Short and long run demand responses
Supply impacts
Financing requirements
Expected impact on key policy objectives
Expected impact on problems
Expected winners and losers
Barriers to implementation

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 transport 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):

• Routes for 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.
• Disabled pedestrians
  Shared space facilities which reduce the distinction between footway and roadway may pose a hazard for blind and partially sighted pedestrians and, potentially, for children.
• Cyclists
  Traffic calming is an appropriate means of facilitating cycling, as lower speeds and flows can lessen the need to separate cyclists from motorized traffic. However, any of the physical means/devices employed to slow traffic have the potential to create problems for cyclists. The latter are more vulnerable to any lack of attention to detail in design of traffic calming measures than are occupants of motor vehicles. Narrowings and pinch points were identified by cyclists as sensitive points because, when carriageway width is reduced, motorists tend to pass cyclists with less clearance (Gibbard et al., 2004. A cycling bypass should be the first option to introduce where a narrowing is introduced on a road. Cyclists also noted that the ramps of some humps and raised intersections can be uncomfortably steep. It this case a smooth slope and a smooth transition between the horizontal and slopping surfaces should be ensured (Local Transport Note 1/07).
• Motorcyclists
  Motorcyclists’ speeds may not be influenced as effectively as those of larger vehicles, and care will be needed to avoid excessive speed. At the same time, it is important to ensure that physical measures do not impose a safety hazard for motorcyclists.
• Routes for emergency service vehicles
  There is concern that the cumulative effect of the growing number of traffic calming schemes could compromise the ability of fire and ambulance service operators to meet the required response times. There have also been suggestions that traffic calming features might unwittingly lead to increased patient discomfort, or cause damage to equipment carried in ambulances or fire appliances (Local Transport Note 1/07). Therefore, it is of high importance 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 on main roads 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 livability 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, or children are taken (on foot or by bike) to the neighborhood schools.
		By providing a safe and comfortable, pleasant environment 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.

	= Weakest possible response,		= strongest possible positive response
	= Weakest possible negative response,		= strongest possible negative response

= No response

Short and long run demand responses

It is unlikely that there will be significant change in demand response of traffic calming over time.

Demand responses
Responses		1st year	2–4 years	5 years	10+ years
	-
	-	/	/	/
	Change job location
	Shop elsewhere (preferably in the local area) neighborhood)
	Go on foot/bike to school in the neighborhood
	Compress working week
	Trip chain
	Work from home
	Shop from home
	Ride share
	Public transport
	Walk/cycle
	-
	-

	= Weakest possible response,		= strongest possible positive response
	= Weakest possible negative response,		= strongest possible negative response

= No response

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. Costs for different types of measure have been quoted over a 30 year period and, in the absence of current data, are cited below without adjusting them for inflation.
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).
 
The following table provides an indicative cost of road humps in 2007. The costs given for round-top and flat-top humps are based on those with tapered edges, so that additional drainage is not required. A kerb-to-kerb flat-top hump suitable for a pedestrian crossing would cost more because drainage is required. Traditional blockwork materials suitable for conservation areas can cost more, and any narrowing of the road would add further to the cost. Gully costs can be significant, in the order £1000. (Local Transport Note 1/07, p.57)

The cost of rumble devices schemes can vary with the type of device and the number of strips or bands used. From the schemes studied it would appear that the typical range for a thermoplastic installation was £500–1500, at 1993 prices. Coarse aggregate rumble areas cost £2500–10,000 at 1992 prices and had an estimated life of about three years. Rumblewave surfacing costs c. £50 per square metre or c. £5000 for a 20 metres strip on a single carriageway road. (Local Transport Note 1/07, p.69)
The following table provides sample US 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.

	= Weakest possible response,		= strongest possible positive response
	= Weakest possible negative response,		= strongest possible negative response

= No response

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.

	= Weakest possible response,		= strongest possible positive response
	= Weakest possible negative response,		= strongest possible negative response

= No response

Expected winners and losers

Winners and losers
Group	Scale of contribution	Comment
Large scale freight and commercial traffic		Where reduction of speed results in increased delay on routes used by freight vehicles, reducing utilisation of freight vehicles making high value journeys.
Small businesses		Where accessibility falls in some local areas, but sometimes economic activity may be improved by pedestrianisation.
High income car-users		Where they may suffer from reduced accessibility.
People with low income car users		Where they may suffer from reduced accessibility.
People with poor access to public transport users		-
All existing public transport users		Where road humps may make bus passengers uncomfortable.
People living adjacent to the area targeted		Where reducing speed and volume of traffic inside the area can improve safety and environments, but the outside will suffer from diverted traffic.
Cyclists including children	/	By reducing speeds and improving street layout.
People at higher risk of health problems exacerbated by poor air quality		By reducing pollution.
People making high value, important journeys		These journeys will have higher values of time and may continue to be made by car, but may be subject to more delay due to reduced accessibility.
Average car users		Where they may suffer from reduced accessibility.

	= Weakest possible response,		= strongest possible positive response
	= Weakest possible negative response,		= strongest possible negative response

= No response

Barriers to implementation

Scale of barriers
Barrier	Scale	Comment
Legal		There are no obvious legal barriers to the implementation of traffic calming.
Finance		Traffic calming measures can be implemented with low costs basically, although for area wide treatment their cost might be significant.
Governance		Traffic calming can usually be implemented by a single authority.
Political acceptability		Decrease of accessibility can be controversial for the residents within the treated area, and diversion of traffic for those outside.
Public and stakeholder acceptability		Acceptance by the local community and cooperation of relevant institutions is the key feasibility issue. Aesthetics are often an important influence on acceptance.
Technical feasibility		There are no obvious technical barriers.

= minimal barrier,

= most significant barrier