Accident Remedial Measures

This measure was fully updated by INSTITUTE FOR TRANSPORT STUDIES (ITS) in 2014 under the CH4LLENGE project, financed by the European Commission.

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A wide range of accident remedial measures exist, both direct and indirect. The latter includes more general purpose policy instruments such as vehicle taxation, land use planning, road design and road furniture. These however, are covered by other KonSULT instruments and lead us to concentrate on direct measures which can be classified under the three following categories,

• Speed limitation;
• Speed enforcement; and,
• Road marking.

Speed limitation can be introduced by legal and/or physical measures. Most countries have general and signposted speed limits stating the highest permitted driving speed on a road. It can also be achieved through traffic calming.

Enforcement of speed limits uses a combination of stationary and mobile methods. The former tend to take the form of ‘speed cameras’ and allow violations of traffic regulations to be detected and registered without law enforcement officers being physically present at the time and place where the traffic violation takes place. The latter rely on police patrols to identify offenders.

Road markings are intended to give drivers reference points with regard to the proximity of their vehicle to other vehicles and the road. Delineator posts and distance markings on motorways, and raised pavement markers can all be used to direct traffic by indicating the path of the carriageway and warn road users about specific hazardous conditions related to the road alignment etc.
The implementation of accident remedial measures can vary from country to country and by situation. Decisions tend to hinge on:

1. What measures to implement - sometimes only single measures may be implemented, whilst at other times a combination of measures will be used;
2. Where to implement them – in a very specific ‘accident black spot’ or across a widespread area (zone approach).

Studies have shown that the introduction of speed limitation and enforcement measures have the greatest potential for improving road safety. They are also the most cost-effective measures. The impact of these measures upon traffic demand and supply has not been specifically analysed. There is a feeling however that the response can be mixed and very much dependent upon existing traffic conditions. Other measures which can provide smaller but important benefits are ITS-systems, driver training and public education and information campaigns.

Terminology

There exists a wide range of accident remedial measures, many of which also have impacts on the challenges KonSULT deals with. A broad spectrum of road safety measures are considered in the Elsevier Handbook of Road Safety Measures (Elvik et al. 2009). These include: General purpose policy instruments e.g. vehicle taxation and land use planning, road design and road furniture, road maintenance, traffic enforcement, vehicle inspection, driver training and licensing, public education and information campaigns and police enforcement and sanctions.

In this section we have narrowed accident remedial measures to the following management measures, not included in other sections of KonSULT:

• Speed limitation
• Speed enforcement
• Road marking

Speed limitation can be introduced by legal and/or physical measures. Most countries have general and signposted speed limits stating the highest permitted driving speed on a road. Speed reducing devices are described in detail in traffic calming and pedestrian crossings.

Speed enforcement includes stationary methods (using radar and similar devices) and speed and behavioural enforcement using "mobile "methods or surveillance/police patrols. Speed cameras generally involve violations of traffic regulations being detected and registered and the vehicle/driver identified automatically - i.e. without police officers being physically present at the time and place where the traffic violation occurs.
Road markings cover the following measures; longitudinal lines on the road surface made of retro-reflective paint or plastic, shoulder rumble strips (edge lines), two-way left turn lanes, raised pavement markers, delineator posts and distance markings on motorways. One can also use combinations of several types of road markings.

Single measures and strategic policies

Measures to increase road safety will often be linked together. Important strategic policy dimensions, including several different measures, are:

• Measures implemented on the basis of Risk analysis vs. Events records
• Blackspot measures on certain places vs. Mass action measures in the road system at large
• Vision Zero vs. Benefit-Cost or Cost effectiveness strategies

Due to the large number of traffic accidents, most countries can base their traffic safety actions or policy on event records i.e. accident registers. In towns and cities, there is a tendency for traffic accidents to cluster at specific places, often at intersections. Increases in accidents at a specific spot may partly be due to inappropriate road design or inappropriate traffic enforcement at that place. Studies give varied results, but overall they indicate that the treatment of both black spots and black sections reduce the number of accidents at the treated sites (Elvik et al. 1997, Elvik & Vaa 2003, Geurts and Wets 2003). Analysis of blackspots and the impact of remedial measures at these points is complicated by a number of factors including regression to the mean (see Maher and Mountain 2009). The effect of black spot treatment on the environment and on demand depends on the measures used.

Norway and Sweden have for a long time been regarded as leading countries in road safety. Governments in both countries have defined Vision Zero as the long-term target for transport safety. Vision Zero aims for a transport system in which nobody is killed or seriously injured. Elvik (1999, 2000) has studied the potential of accident reduction of various road safety strategies. The strategies shown in Table 1 consist of different packages of measures, e.g. “business as usual” means a strategy where one will continue to use the safety measures used today. Table 2 shows that all alternatives to “business as usual” will reduce the annual number of road accident fatalities. According to this table, the costs of road safety measures are lower in a benefit cost strategy than in a “business as usual” strategy. It is, in other words, possible to improve road safety substantially without increasing current public spending on road safety measures.  Further, while a Vision Zero strategy may be expensive, there is work suggesting that the impact of a Vision Zero (VZ) policy can mean that “a non-strict version of the VZ may actually from an economic point of view be part of an optimal second-best strategy” (Johansson-Stenman, 2000).

Table 1: Summary of effects of alternative road safety strategies in Norway and Sweden. Annual number of road accident fatalities. The benefits include the total monetary benefits for safety, mobility and the environment. Amounts in million NOK. 1 NOK = 0.13 US Dollars. (Source: Elvik 2001)

	Alternative road safety strategies (ten year period 2002-2011)
Type of impact	Business as usual	Benefit cost	Vision Zero	Maximum potential
NORWAY
Road accidents fatalities	332	183	148	118
Benefit/cost ratio	0.64	1.79	0.47	0.28
Public expenditures (mill per year)	2,152	2,075	9,617	15,728
Private expenditures (mill per year)	694	1,729	2,758	30,491
Net effect on mobility	Positive	Positive	Negative	Negative
Net effect on the environment	Negative	Positive	Positive	Positive
SWEDEN
Road accidents fatalities	528	316	230	180
Benefit/cost ratio	0.22	1.25	-0.02	0.10
Public expenditures (mill per year)	5,717	3,357	17,983	32,218
Private expenditures (mill per year)	1,560	3,647	4,924	11,466
Net effect on mobility	Negative	Negative	Negative	Negative
Net effect on the environment	Negative	Positive	Positive	Positive

Why introduce accident remedial measures?

Traffic accidents create an enormous global health problem.  According to the World Health Organisation report there are more than 1.24 million road deaths worldwide each year, and it is estimated that there are 20 times as many non-fatal road injuries (2013, pp. 4-7).  Risks of death and serious injury on the roads vary greatly within and between countries, and across users of different transport modes, among different income groups, and among different age groups.    

The WHO (2013) reports that young people are especially vulnerable to road traffic deaths, with this being the “leading cause of death for young people aged 15–29 years” (p. 1).   Further the WHO study found that “[h]alf of the world’s road traffic deaths occur among motorcyclists (23%), pedestrians (22%) and cyclists (5%) – i.e. “vulnerable road users.”” (p. 6), and that the ”European Region has the highest inequalities in road traffic fatality rates, with low-income countries having rates nearly three times higher than high-income countries (18.6 per 100 000 population compared to 6.3 per 100 000)” (2013, p. 5).

The size of the health risk posed by road accidents in a country depends on the amount of travel performed per year per inhabitant, the level of traffic risk, and the resources available to protect road users from fatal injury or provide rapid medical treatment of serious injuries. There is a simple definitional relationship between health risk and traffic risk (Elvik & Vaa 2003):

Health risk = Traffic risk x Motorization rate (the number of motor vehicles per inhabitant in a country)

There are fairly large variations in both health risk and traffic risk between motorised countries (having at least 0.14 motor vehicles per inhabitant).

Table 2: Road accident fatalities, health risk and traffic risk in IRTAD member countries. Source: IRTAD (OECD)

Country	Year	Number of road accident fatalities	Health risk (killed per 100,000 inhabitants)	Traffic risk (killed per 100,000 motor vehicles)
Australia	2000	1818	9.5	15.3
Austria	2000	976	12.0	19.1
Belgium	2000	1470	14.2	25.6
Canada	1999	2972	9.7	16.6
Czech Republic	2000	1486	14.5	31.8
Denmark	2000	498	9.3	20.7
Finland	2000	396	7.7	15.9
France	2000	8079	13.6	23.6
Germany	2000	7503	9.1	14.6
Greece	1999	2116	20.2	41.8
Hungary	2000	1200	11.9	44.3
Iceland	2000	32	11.3	17.6
Ireland	2000	415	11.0	24.6
Italy	2000	6410	11.1	16.9
Japan	2000	10403	8.2	13.2
Luxembourg	2000	76	17.5	23.8
Netherlands	2000	1082	6.8	13.6
New Zealand	2000	462	12.1	17.8
Norway	2000	341	7.7	13.4
Poland	2000	6294	16.3	44.6
Portugal	2000	1860	19.6	23.5
Republic of Korea	2000	10236	21.8	78.4
Spain	2000	5776	14.6	24.8
Sweden	2000	591	6.7	12.5
Switzerland	2000	592	8.3	12.9
Turkey	2000	5123	7.5	53.6
United Kingdom	2000	3580	6.0	12.1
United States	2000	41821	15.2	19.3
Total IRTAD		123608	12.2	20.6

Road accident costs have three major components; direct costs (medical treatment, repairing vehicles etc.), indirect costs (losses in output attributable to premature death, permanent impairment or temporary absence from work) and the value of preventing premature death and pain, grief and suffering caused by road accidents. If an economic valuation of lost quality of life is included, the costs amount to 2.5% of GNP (gross national product) on the average, based on estimates for 12 countries. There is substantial variation among countries, with a range from 0.5% to 5.7%. If the costs of lost quality of life are excluded, road accident costs on the average drop to 1.3% of GNP, with a range from 0.3% to 2.8% (Elvik 2000).

Which measures should be introduced?

A whole range of measures is necessary to maintain traffic safety levels, and to increase traffic safety further. The greatest potential for improving road safety is attributable to traffic enforcement (particularly speed limits), motor vehicle safety standards and police enforcement. The benefits of increasing speed enforcement greatly exceed the costs. Improvements to infrastructure can impact on safety, however such improvements can prompt greater risk taking by drivers (Albalate et al. 2013).  Motorways are the safest type of road, but building them is an expensive way of reducing accidents and can have severe detrimental impacts for social equity, particularly exclusion of those without access to motor vehicles, as well as risks of increasing greenhouse gas emissions and poor local air quality by inducing travel demand. ITS-systems cannot improve road safety in the short term, but could contribute in the longer term (15-20 years). Driver training and public education and information campaigns have a much smaller potential for improving road safety (Elvik 1999).

Why introduce speed limitation and enforcement?

High speeds and great variations in speed increase the probability of accidents and serious personal injuries, since the demands on the road users’ observations and reactions increase, and because braking distance increases proportionally with the square of the speed. Furthermore, the risk of fatal injuries increases by the square of the change in speed to which the body is exposed in an accident. This means that for a given increase in speed, the risk of fatal injury increases even more.  Recent studies support evidence that any reductions in speed (and impact speed) are beneficial in reducing risks of fatality for pedestrians hit by vehicles (Kröyera at al. 2014)

In figure 1 the relationship between changes in the number of accidents and changes in mean speed are presented. Both variables are given as percentage changes.

Figure 1: Relationship between changes in the number of accidents and changes in speed. Percentage. Source: Elvik et al 1997. Copyright ©TOI

A driver may want to get from A to B using as little time as possible and with a reasonable feeling of safety while travelling. Drivers might trade off travel time against safety for themselves and for other road users. In this evaluation, factors such as road geometry, driving and light conditions, the amount of traffic, the characteristics of the car, drivers’ perceptions of their own skills and motives, accident rates and the possibility of police enforcement are all included.

The objective of speed-reducing devices, which make fast driving uncomfortable or impossible, is to get speeds down to a desired level. Speed-reducing devices are intended to force vehicles to keep to low speeds, so that the risk of accidents is reduced and feelings of safety increase.

Respect for the speed limit may vary considerably amongst car drivers. Exceeding the speed limit is probably the most common form of violation of traffic regulations. Many drivers drive faster than the legal speed limit - some much faster. Even though the actual risk of apprehension may be very low, the knowledge that speeds may be checked by the police does affect behaviour. The objective of speed enforcement and enforcement is to maintain and possibly also increase respect for the legal speed limits.

Why introduce road marking?

To drive safely and comfortably, drivers depend on reference points in the proximity of the vehicle and further ahead in the direction they are driving. Road markings are intended to give drivers such reference points. By road marking one can direct traffic by indicating the path of the carriageway, warn road users about specific or hazardous conditions related to the road alignment, control traffic, for example by reserving certain parts of the road for certain road user groups as well as supplement and reinforce information given by means of traffic signs.

Demand impacts

It is documented that speed limitation and enforcement will induce increased travel/time costs for vehicles (Elvik et al. 2009). On the other hand, if speed becomes more evenly distributed in areas where traffic is heavy, this may lead to better utilised capacity and reduce time costs for the society at large.  Studies report that in addition to increasing risks to pedestrians and cyclists, high traffic speed can inhibit levels of walking and cycling (Jacobsen et al. 2009; Pucher and Buehler 2008). 

Suspension of the driving licence for speeding will make it necessary for the offender to shift to other transport modes for a period. The impacts for these drivers are not commented on.

Road marking will not have demand impacts of the type discussed below. (Rumble strips, though, will increase noise.) The content in the tables therefore only refers to speed limitation and enforcement.

Responses and situations
Response	Reduction in road traffic	Expected in situations
		Speed limitation and enforcement will reduce speed levels and can force some drivers to change departure time.
	/	Reduced speed on roads in urban centres, living areas etc. might stimulate some drivers to avoid driving through such areas and choose routes and highways with higher speed limits. This will mostly give longer trips, i.e. increase in km by car.
	/	Some drivers might change destinations for activities where alternatives are possible, like shopping etc. Lower speeds encourage greater walking and cycling to destinations, motor traffic might be expected to decrease. The outcome on kilometres will depend on the alternative of the activity.
		Trips by motor vehicle might be partly replaced by walking and cycling.
		Motor traffic might be partly replaced by walking and cycling.
		As above.
		It is possible that areas with reduced traffic speed are more attractive to live in.

	= Weakest possible response		= Strongest possible positive response
	= Weakest possible negative response		= Strongest possible negative response
	= No response

Short and long run demand responses

It might take some time to adjust to a changed traffic pattern and to change habits to adapt to new speed limitations.

The immediate effects of speed enforcement will be maintained from 2 days to 10 weeks (Elvik et al 1997), but will also hopefully lead to increased respect of legal speed in general.

Demand responses
Response	-	1st year	2-4 years	5 years	10+ years
	-
	-	/	/	/	/
	Change job location	/	/	/	/
	Compress working week
	Ride share
	-
	-

	= Weakest possible response		= Strongest possible positive response
	= Weakest possible negative response		= Strongest possible negative response
	= No response

Supply impacts

Speed limitation and enforcement will not influence the road space at hand or its capacity. Road marking aimed at reserving certain parts of the road for certain traffic groups, will alter the supply between different road users.

The throughput of a road is at its highest if a speed of around 60 km/h is enforced. If in addition such speed enforcements induce more evenly distributed individual speeds, the throughput is increased further.

Financing requirements

The costs of speed limitation and roadmarking depend on the measures used and the amount of traffic on the road.  Police enforcement is more expensive, but the benefits largely exceed the costs, and this ratio may increase when wider social benefits including health and social inclusion are considered. 

Expected impact on key policy objectives

Road markings have no impacts on other key policy objectives other than safety. Their impacts depend on the type of road marking, cf. Evidence of performance. The comments below refer to speed limitation and enforcement.

The impact on environment is a complex issue, since lower speed has different impacts on noise and pollution, as well as on various type of pollution, cf. Evidence on performance.

Contribution to objectives
Objective	Scale of contribution	Comment
		There will be minor improvements in the efficiency of use of the road network both because there are fewer accidents and resulting disruptions and because traffic flows are more uniform.
		Lower speeds in living areas will contribute, e.g. make walking less dangerous and pedestrian crossing easier.
	/	Speed reduction can reduce energy consumption and greenhouse gas emissions.  There is some uncertainty about the impact on air quality of low speeds. However if lower speeds encourage walking and cycling this will benefit the environment.
		The WHO (2013) describes how road traffic accidents disproportionally impact on vulnerable groups.  Lower speed limits can encourage walking and cycling bringing benefits of affordable mobility, improved health through active transport, improved social interaction and participation in society.
		Lower speed is the most efficient way to reduce the number and severity of traffic accidents. Speed limitation and enforcement are more efficient than road marking.
		There may be economic benefits from congestion reduction if road traffic decreases and walking and cycling increase. Reducing the high mortality burden that road traffic places on young and economically active adults may support economic development (see WHO 2013)
		Finance is required to implement measures however this is offset by economic savings from accident reduction and from benefits of mode change.

	= Weakest possible positive contribution		= Strongest possible positive contribution
	= Weakest possible negative contribution		= Strongest possible negative contribution
	= No contribution

Expected impact on problems

For many problems speed reduction can have both positive and negative effects.

Contribution to alleviation of key problems
Problem	Scale of contribution	Comment
Congestion-related delay	/	Lower speeds induce individual delays and congestion, but if speed becomes more evenly distributed, this may mean that the capacity is better utilised when traffic is heavy. If walking and cycling increase as a result of lower speed, this can reduce congestion.
Community impacts		By making it easier, less dangerous to cross roads for pedestrians and cyclists. Reduced severance and increased liveability.
Environmental damage	/	Speed reduction at high speeds will reduce energy consumption and CO2, but at lower speeds the effect is opposite. However if lower speed encourage walking and cycling this will benefit the environment. Lower speed will reduce recirculation of dust particles. No evident positive effect on other local pollutants. If increased congestion with uneven (transient) driving, local emissions will also increase. However if lower speed encourage walking and cycling this will benefit the environment.
Poor accessibility		Lower vehicle speed in living areas will make travelling easier for pedestrians and cyclists, many of whom are children, older people and not car-owners.
Disproportionate disadvantaging of particular social or geographic groups		See above.
Number, severity and risk of accidents		Reducing speed will reduce accident risk and the severity of accidents substantially.
Suppression of the potential for economic activity in the area		There may be economic benefits from congestion reduction if road traffic decreases and walking and cycling increase. Reducing the high mortality burden that road traffic places on young and economically active adults may support economic development (see WHO 2013).

	= Weakest possible positive contribution		= Strongest possible positive contribution
	= Weakest possible negative contribution		= Strongest possible negative contribution
	= No contribution

Expected winners and losers

It is difficult to see how there can be any losers if policies of land use density and mix are wisely applied. This is because there should be a wider range of destinations within a given distance and public transport operations are made easier. There is no discouragement per se to any motorised mode: the reduction in traffic by reducing the need to travel will in fact benefit all motorised modes.

Winners and losers
Group	Winners/Losers	Comment
Large scale freight and commercial traffic		More evenly distributed speed and better capacity on highways will give advantages to commercial traffic.
Small businesses		More evenly distributed speed and better capacity on highways will give advantages to commercial traffic.
High income car-users		On the average accident reductions outweigh time losses.
People with a low income		Given better conditions for pedestrians and cyclists.
People with poor access to public transport		At specific spots, easier crossing for pedestrians might increase access to public transport stations.
All existing public transport users		-
People living adjacent to the area targeted	/	Better conditions for pedestrians and cyclists are positive effects, but possible congestion is a negative one.
Cyclists including children		Lower speed reduces risk of death and serious injury to cyclists. By improving perceptions of safety, it might increase cycling bring health benefits of active travel and social inclusion and economic inclusion for those withuot vehicles.
People at higher risk of health problems exacerbated by poor air quality	/	Lower speed will reduce recirculation of dust particles. No evident positive effect on other local pollutants. If increased congestion with uneven (transient) driving, local emissions will also increase. However if lower speeds encourage mode shift to walking and cycling this will benefit air quality.
People making high value, important journeys	/	Depends on the route necessary. Individual delays will affect this group more than the average car user, but so will more even speed on highways and fewer accidents.
The average car user	/	Individual delays will be a negative impact, (as will suspension of drivers licence), but more even speed on highways and fewer accidents will be positive ones.

	= Weakest possible benefit		= Strongest possible positive benefit
	= Weakest possible negative benefit		= Strongest possible negative benefit
	= Neither wins nor loses

Barriers to implementation

Speed limitation and especially speed enforcement encounter great barriers to implementation due to the strong position of car owners and car organisations in the political process.

Scale of barriers
Barrier	Scale	Comment
Legal		None.
Finance		Finance is required to implement measures however this is offset by economic savings from accident reduction and from benefits of mode change.
Governance	/	There may be no barrier although co-ordination between different levels of government and different public services and enforcement agencies may be required.
Political acceptability	/	Political acceptability can echo aspects of (perceived) voter acceptability.
Public and stakeholder acceptability	/	There can be opposition from some drivers, however there is broad support from accident prevention and health organisations, and from environmental, and social justice groups and residents.
Technical feasibility		None.

= Minimal barrier

= Most significant barrier

Impacts of 20 mile per hour zones in London

The London School of Hygiene & Tropical Medicine have undertaken two studies of the impacts on collisions and casualties road of 20 mile per hour zones in London. One report focuses on the overall impacts and the other on the impacts on inequalities in the city (Grundy et al. 2008a; Grundy et al. 2008b). The studies were promoted by perceived need for robust evidence on the impacts of the 20mph zones which: ‘in London [have] increased year on year since they were first introduced in 1990/91, to a total 399 zones by 2007/08, with some Boroughs far more enthusiastic about adoption than others’ (2008a, p.5). The methodology of the studies is described in detail in the reports, and took account of characteristics of each 20mph zone and the impacts of the zone over time.

The studies analysis found “a 42% reduction (95% CI 36%, 48%) in all casualties within 20 mph zones compared with outside areas, adjusting for an annual background decline in casualties of 1.7% on all roads in London. The largest effects of 20 mph zones were found for all casualties aged 0 -15 killed or seriously injured (KSI) and for car occupants. A reduction was evident for all outcomes examined. In areas adjacent to 20 mph zones, reductions compared with outside areas were evident for most outcomes, except for those killed”  (2008a, p.6).

“The effects of 20 mph declined over time, although those implemented in the most recent years (2000-2006) still had an effect of reducing all casualties by 23% (95% CI 15%, 30%) within the 20 mph zone, and 3% (95% CI -1%, 7%) in adjacent areas,” (2008a, p.7). The study indicated “some evidence that 20 mph zones are more effective in reducing KSI casualties in less deprived areas compared to more deprived areas” (2008a, p.7).

Contribution to objectives
Objective	Scale of contribution	Comments
		-
		-
		-
		The London study found “some evidence that 20 mph zones are more effective in reducing KSI casualties in less deprived areas compared to more deprived areas” (2008a, p.7).  This should be considered alongside matters of whether more deprived areas have higher casuality rates and so begin from a position of greater disadvantage. Further it should be considered alongside evidence on inequalities in rates of KSI faced by users of different travel modes, and potential benefits to those groups of lower speeds.
		-
		-
		The London 20mph studies found positive benefit-cost ratios for introduction in zones with over 0.7 casualties per year per km.

	= Weakest possible positive contribution		= Strongest possible positive contribution
	= Weakest possible negative contribution		= Strongest possible negative contribution
	= No contribution

Meta analysis of case studies

The Handbook of Road Safety Measures (Elvik & Vaa 2003), provides a comprehensive survey of road safety measures, includes meta-analysis of a large number of studies on the effects of different speed limitation and enforcement measures as well as road markings. The meta-analysis assigns statistical weights to studies by sample size and sorts them by design quality and thus gives the most systematic overview of impacts, especially on traffic safety. The evidence of performance described is mainly based on this information. For environmental impacts the TØI Environmental Handbook (Kolbenstvedt et al 1999) is used as well.

Context

The studies included are from several countries; e.g. Australia, Canada, Denmark, Finland, Germany, Great Britain, Norway, Sweden, Switzerland, and USA.

Impacts on demand

The measures in this area are not primarily intended to affect transport demand, and most impact studies concern effects on speed and accidents. Indirect impacts on demand will have to be derived from knowledge about speed impacts.

Reductions in speed limits and transition from unrestricted speed to speed limits have been introduced in a number of countries. On average a reduction of the legal speed limit with 10 km per hour will result in a speed reduction of 3 km per hour. Though many drivers exceed the speed limits, implementing lower speeds increase the time taken to travel and transport goods. By reducing speed from for example, 80 to 70 km per hour for a 60 km trip, travel time increases from 45 minutes to around 51 minutes.

Physical speed-reducing devices reduce speed. This can induce individual vehicle delays and can deter traffic, especially heavy vehicles. It has not been shown that these effects always occur. On a typical access road with a length of up to 0.5 kilometres, a reduction in speed from 35 km per hour to 25 km per hour will lead to a delay of a maximum of 20 seconds per car. It is not known whether humps create problems for winter maintenance of roads.

It has also been shown that traffic volume goes down on roads where humps are constructed (e.g. Webster & Mackie 1996). On average, the reduction in traffic is around 25% (-33%; -14%). This indicates that the actual roads had a certain amount of through traffic before the humps were constructed.

Stationary and automatic camera speed enforcement affects the speed level and gives an average speed reduction of around 2 km per hour. The halo effects in time and space show that the reductions can be maintained from 2 days up to 10 weeks after increases in enforcement procedures end. The distance-halo effect which has been demonstrated, varies between around 1 km and 22 km from the point where the stationary enforcement took place (Ragnøy 2002). It is not known to what extent drivers compensate with higher speeds outside areas which they suspect are monitored manually or automatically.

Speed camera can to some extent give raise to "kangaroo driving". This may interfere with the flow of traffic, but the effect of this measure on demand and travel choices is not sufficiently known.

Figure 2: Longitudinal speed profile E6 Hedmark. Speed limit 90km/h. Relative average speed before and after speed cameras in km/h. (Adjusted for changes in comparison sites). Change in average speed in km/h. Copyright ã TOI

Drivers who have had their driving licence withdrawn due to speed enforcement, will have their individual mobility reduced for as long as the driving licence is withheld.

Effects of road marking on speed vary, and the results differ. Both decreased and increased speed are found.

Impacts on Supply

Speed limitation and enforcement does not occupy road space, i.e. capacity.

Lower speeds may have an effect in that the average speed on the actual road is reduced, thus increasing travel time for the individual, assuming that there are no capacity problems. Lower speed can also induce congestion when capacity is limited. However, reducing average speed can also lead to less dispersion and a more even level of speed, so that the flow of traffic eventually improves and the road capacity can be better utilised. This is especially important when traffic is heavy.

Road marking aimed at reserving certain parts of the road for certain traffic groups will alter the supply between different road users.

Other Impacts – Traffic safety

The objective of speed limitation and enforcement is to reduce traffic accidents. Meta-analysis of some hundreds of studies, clearly show that this objective is achieved (Elvik &Vaa 2003). If we look at speed limitation it may be concluded that a majority of the results show reductions in the number of traffic accidents. Very often, these reductions are also statistically significant. Furthermore, these measures are the most cost-effective traffic safety measures of all (Elvik 1999 and 2000). On the contrary roadmarking measures have no statistical significant effect on accidents.

Taking all types of accidents and levels of severity together, stationary speed enforcement is associated with a 2% reduction in accidents, while speed cameras give a reduction of 19 %, both statistically significant, see table 3. As to stationary speed enforcement the effect is largest for fatal accidents, which are reduced by 14%. Speed cameras appear to have a greater effect in densely populated areas (28% reduction) than in sparsely populated areas (4% reduction). The area of effect is limited to the road where speed cameras are installed.

Table 3: Best estimate and confidence interval for accidents of stationary and automatic speed enforcement. Percentage change in the number of accidents. (Source: Elvik & Vaa 2003)

	Percentage change in the number of accidents
Accident remedial measure	Types of accidents affectedseverity, area type	Best estimate	95% Confidence interval
Stationary speed enforcement	All	- 2	(- 4; - 1)
	Fatal accidents	- 14	(- 20; - 8)
	Injury accidents	- 6	(- 9; - 4)
	Property damage only accidents	+ 1	(- 1; + 3)
Automatic speed enforcement (ATE)	All	- 19	(- 20; - 18)
	Injury accidents	- 17	(- 19; - 16)
	All accidents in densely populated areas	- 28	(- 31; -26)
	All accidents in sparsely populated areas	- 4	(- 6; - 2)

Table 4 shows that humps reduce the number of injury accidents, with a given amount of traffic, by around 50 %. Rumble strips and speed zones also have a significant positive effect, around 30% on accidents. The majority of results come from simple before- and after- studies, which have not controlled for regression-to-the-mean in the number of accidents. On the other hand, a number of studies have measured changes in both the amount of traffic and speed levels in roads where the measures have been introduced.

Table 4: Effects on accidents of speed-reducing devices. Percentage change in the number of accidents. (Source: Elvik & Vaa 2003)

	Percentage change in the number of accidents
Accident remedial measure	Types of accident affectedSeverity and place	Best estimate	95% Confidence interval
Humps - effect on roads with humps	All injury accidents	-48	(-54; -42)
Humps - effect on surrounding roads	All injury accidents	-6	(-9; -2)
Raised intersections (plateau intersections)	Injury accidents at intersections	+5	(-34; +68)
	Property damage accidents only at intersections	+13	(-55; +183)
Rumble strips (especially in front of intersections)	Injury accidents at intersections	-33	(-40; -25)
	Property damage accidents only at intersections	-25	(-45; -5)
	Accidents at intersections - unspecified severity	-20	(-25; -5)
Speed zones (30 km per hour (20 mph) zones in residential areas, with humps)	All injury accidents	-27	(-30; -24)
	All property damage only accidents	-16	(-19; -12)

The main impression from the meta-analysis in the Handbook of Road Safety Measures (Elvik & Vaa 2003) is that the majority of road marking measures appear to have relatively little effect on the number of accidents. Changes in the number of accidents are in many cases not greater than + / - 5% and are, as a rule, not statistically significant. The exception to this rule are profiled edge lines, which appear to reduce the number of driving off the road accidents by around 30%, and distance markers on motorways, which reduce the number of accidents by more than 50%. The idea of distance markers is to help car drivers maintain an adequate distance from those in front. A combination of several road marking measures appears to have a more favourable effect on the number of accidents than individual road marking measures, see table 5.

Table 5: Effect on accidents of different effective road marking measures. Percentage change in the number of accidents. (Source: Elvik &Vaa 2003)

	Percentage change in the number of accidents
Accident remedial measure	Types of accident affected severity, place	Best estimate	95% Confidence interval
Profiled edge line (shoulder rumble strip)	All injury accidents	+2	(-17; +26)
	Driving off the road accidents, Unspecified degree of injury	-31	(-45; -15)
Distance markers (angle symbols) on motorways	Injury accidents on motorway	-56	(-76; -19)
Edge lines and background / directional markings in curves	All injury accidents	-19	(-46; +23)
Combination of edge lines and centre lines	All injury accidents	-24	(-35; -11)
Combination of edge lines and centre lines	All injury accidents	-45	(-56; -32)

Explanations for these results are little known. However, a number of studies have shown that different types of road markings can lead to higher speeds, see the paragraph on the effect on mobility.

Other Impacts – Environment

The environmental effects of road traffic depends, among other things, on the amount of traffic, the speed, the variation in speed, the composition of the traffic, the road alignment and the road surroundings. A significant change in environmental effects can be achieved by changing these conditions. Measures that improve the quality of traffic flow, i.e. which reduce queuing problems and lead to more even speeds, normally reduce the environmental problems along a road. The same is true of measures that reduce the amount of traffic.

Speed reductions however, have both positive and negative impacts on the environment.

Measures that reduce speed will in general have a favourable effect on the noise level. Specific measures like Rumble strips can however increase the noise level by 2-6 dBA. The increase in noise levels will be lowest for paving stones and highest for grooves in the road surface.

The global effects are related to energy consumption and greenhouse gases. Speed reduction at high speeds will reduce energy consumption and CO2, but at lower speeds the effect is opposite. For light traffic the energy consumption per km is high when starting, and decreases up to speeds of 40 km per hour. At speeds of 70 –80 km per hour the wind resistance will again increase energy consumption. Heavy vehicles have the same pattern, but the energy consumption will increase already form about 50 km per hour.

Local environmental impacts depend most strongly of the car’s age, driving style, cold starts and are also to some extent dependent on local geography. Speed is a less important factor below speed of 120 km pr hour. The clearest positive impact on pollution is that lower speed will reduce recirculation of dust particles (Amundsen & Ragnøy 2002). For modern cars no evident positive effects of lower speed as such are found on other local pollutants. Catalyst cleaning of exhaust from modern gasoline fuelled cars is not dependent on speed (SSB & SFT 1999).

If increased congestion involves variable driving speeds, local emissions will increase. More even speeds will reduce emissions. Driving pattern and transient driving have stronger environmental effects than the speed as such. The impacts of speed limitations on the driving pattern is less known.

Contribution to objectives

Case study confirms wider assessments of the measures.

Contribution to objectives
Objective	Scale of contribution	Comment
		The benefits exceed the costs by far, all impacts included. The value of accidents can influence the efficiency contribution.
	/	The benefits exceed the costs by far, all impacts included. The value of accidents can influence the efficiency contribution.
		Lower speeds in living areas will contribute, e.g. make walking less dangerous and pedestrian crossing easier.
		Speed reduction can reduce energy consumption and greenhouse gas emissions.  There is some uncertainty about the impact on air quality of low speeds. However if lower speed encourage walking and cycling this will benefit the environment.
		The WHO (2013) describe how road traffic accidents disproportionally impact on vulnerable groups.  Lower speed limits can encourage walking and cycling bringing benefits of affordable mobility, improved health through active transport, improved social interaction and participation in society.
		Lower speed is the most efficient way to reduce the number and severity of traffic accidents. Speed limitation and enforcement are more efficient than road marking.
		There may be economic benefits from congestion reduction if road traffic decreases and walking and cycling increase. Reducing the high mortality burden that road traffic places on young and economically active adults may support economic development (see WHO 2013).

	= Weakest possible positive contribution		= Strongest possible positive contribution
	= Weakest possible negative contribution		= Strongest possible negative contribution
	= No contribution

Contribution to problems

Summary of different systems’ contribution to alleviation of key problems
Problem	Scale of contribution	Comment
Congestion-related delay	/	Lower speeds induce individual delays and congestion, but if speed becomes more evenly distributed, this may mean that the capacity is better utilised when traffic is heavy. If walking and cycling increase as a result of lower speed, this can reduce congestion.
Community impacts		By making it easier, less dangerous to cross roads for pedestrians and cyclists. Reduced severance and increased liveability.
Environmental damage	/	Speed reduction at high speeds will reduce energy consumption and CO2, but at lower speeds the effect is opposite. However if lower speed encourage walking and cycling this will benefit the environment. Lower speed will reduce recirculation of dust particles. No evident positive effect on other local pollutants. If increased congestion with uneven (transient) driving, local emissions will also increase. However if lower speed encourage walking and cycling this will benefit the environment.
Poor accessibility		Lower vehicle speed in living areas will make travelling easier for pedestrians and cyclists, many of whom are children, older people and not car-owners.
Disproportionate disadvantaging of particular social or geographic groups		Lower vehicle speed in living areas will make travelling easier for pedestrians and cyclists, many of whom are children, older people and not car-owners.
Number, severity and risk of accidents		Reducing speed will reduce accident risk and the severity of accidents substantially
Suppression of the potential for economic activity in the area		There may be economic benefits from congestion reduction if road traffic decreases and walking and cycling increase. Reducing the high mortality burden that road traffic places on young and economically active adults may support economic development (see WHO 2013).

	= Weakest possible positive contribution		= Strongest possible positive contribution
	= Weakest possible negative contribution		= Strongest possible negative contribution
	= No contribution

Appropriate contexts

Appropriate area-types
Area type	Suitability
City centre
Dense inner suburb
Medium density outer suburb
Less dense outer suburb
District centre
Corridor
Small town
Tourist town

= Least suitable area type

= Most suitable area type

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Elvik, R. (1999): Bedre trafikksikkerhet i Norge. (Improving road safety in Norway) TØI rapport 446. Oslo, Transportøkonomisk institutt. (English summary at http://www.toi.no).

Elvik, R. (2000): How much do road accidents cost the national economy? Accident Analysis & Prevention, 32 (6), 849-851.

Elvik, R. (2001): Improving road safety in Norway and Sweden: An analysis of the efficiency of current policy prioritie. Traffic engineering & Control, 42 (1), 9-16.

Elvik, R., Mysen, B.A. & Vaa, T (1997): Trafikksikkerhetshåndbok (Traffic safety handbook). Oslo, Institute of Transport Economics. Complete Norwegian version accessible at www.toi.no

Elvik, R.,  Hoye, A., Vaa  T. and Sorensen M. (2009): The Handbook of Road Safety Measures. NY, Amsterdam, Elsevier

Elvik, R.; Amundsen, A.H. (2000): Improving road safety in Sweden. Main report. TØI report 490. Oslo, Institute of Transport Economics. (Summary at http://www.toi.no)
Global Road Safety Partnership (see homepage: http://www.grsproadsafety.org).

Elvik, R. & Vaa, T (2003): The Handbook of Road Safety Measures. NY, Amsterdam, Elsevier science

Geurts, K. and Wets G. (2003) Black Spot Analysis Methods: Literature Review RA-2003-07 Kennis Verkeersonveiligheid, Accessible at http://www.ctre.iastate.edu/educweb/ce552/docs/Black%20Spot%20analysis%20literature%20%20Review.pdf

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Grundy C, Steinbach R, Edwards P, Green J, and Wilkinson P. (2008b) The Effect of 20 mph Zones on Inequalities in Road Casualties in London: A report to the London Road Safety Unit. London: LSHTM.

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Murray & Lopez (1996): Global Burden of Disease study. Geneva, World Health Organization.

Ragnøy, A. (2002): Automatisk trafikkontroll (ATK) – effekt på kjørefart. (Speed cameras (ATE) – Effects on speed). TØI report 573. Oslo, Institute of Transport Economics, 2000. (English summary at http://www.toi.no)

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Probably the best international database on road accidents is the IRTAD database (International Road and Traffic Data Base), maintained by the German Federal Highway Research Institute (Bundesanstalt für Strassenwesen, BASt) on behalf of the OECD. Key figures from this data base can be accessed from the homepage of the BASt (http://.www.bast.de).