Fact sheet

Speed and speed management

Summary

Many drivers– Intentionally or unintentionally –  go too fast: faster than the speed limit, or too fast for specific conditions such as fog or rush hour volumes. At higher driving speeds the braking distance is longer, response time is shorter and a possible crash is harder. Excess speed increases the likelihood of traffic crashes and the risk of serious consequences. It is estimated that a third of all fatal crashes is (partly) due to driving too fast.

To prevent speeding (speed management), several measures can be taken, among others clear and credible speed limits, speed humps and other physical speed inhibitors, police controls and technological support such as intelligent speed assistance (ISA). A structural improvement of speed behaviour calls for a combination of these and other measures.

Facts

Why is a high driving speed dangerous?

A higher driving speed leads to a greater impact in the event of a crash, which, in turn, results in more serious injury. Moreover, at higher driving speeds there is less time to process information and react, and the braking distance is also longer. This reduces the possibility of avoiding a collision. This can be seen on roads where the speed increases. If on a road the average speed goes up, this leads to a higher risk of crashes with, in addition, a greater risk of a serious outcome (see also What is the road safety effect of speed?). It must be noted that not just the absolute speed plays a role; speed differences also affect safety. Speed differences result in more encounters with other road users and also in more lane changing and overtaking manoeuvres (see also What is the road safety effect of speed differences?).

How often do drivers speed?

The share of speed offenders varies greatly. This is shown by speed measurements that some Dutch road authorities carried out on their roads. They measured that on rural roads (provincial roads in Drenthe, Friesland and Zeeland – not published) an average of around 30% of the vehicles exceed the limit, but percentages of around 80% are no exception. Also on urban roads in built-up areas percentages of offenders above 80% are no exception (see, for example, Van Schagen et al., 2010). The proportion of offenders on a road is strongly related with the credibility of the limit, the traffic volume and the enforcement level.

Speeding is not just about exceeding the speed limit. It is also about inappropriate speed, i.e. a speed that is higher than is safe at that moment, regarding the local conditions (weather, traffic volumes).

What is the road safety effect of speed?

In general, at constant conditions, a speed increase comes with more casualties; a speed reduction with less casualties (Nilsson, 1982; Aarts & Van Schagen, 2006; Elvik, 2009, Elvik, 2013). A speed reduction or speed increase has the greatest effect on the number of road deaths. The effect of a similar change on serious road injuries is somewhat smaller, and on the number of slightly injured somewhat smaller again.

In the 1980s, the effect of an increase (or decrease) of the speed driven on a road was expressed in a formula based on kinetic laws (Nilsson, 1982):

This establishes a relationship between the number of injury crashes and the average speed before and after the speed change. In words: the ratio between the number of injury crashes before and after a change in speed is equal to the square of the ratio between the average speed before and after that change. If we do not look at the effect on all injury crashes, but at the effect on the number of crashes with serious injury, we need to elevate the speed ratio to the power of 3 instead of 2 (square). For the relationship with the number of fatal crashes the power is even as high as 4. This formula of Nilsson means that a speed increase of, for example, 10% will lead up to (approximate and on average) 20% more injury crashes, up to 30% more traffic crashes with serious injury and up to 40% more fatal road crashes.

The mentioned formulas of Nilsson – with the exponents 2, 3 and 4 – are, as has been said, based on the theory of kinetic laws. Over the years, however, a large number of empirical studies into speed and crashes have been carried out. Based on the results Elvik (2009) has specified the exponents — that is the power to which the speed ratio in the formula is raised – in more detail. Table 1 shows the results: the best estimate as well as the range of the value with 95% certainty.

Example: If we want to have an indication of the effect of a change in speed on the number of deaths on rural roads we use the formula:

This results in a 'best estimate'. To know the range of the effect with 95% certainty, we apply this formula twice, namely with the exponents in brackets in the table – in this example therefore the values 4.0 and 5.2.

Table 1. The exponents in the formulas for the relationship between speed and crashes/casualties with different severity (Elvik, 2009).

 

What is the road safety effect of speed differences?

Not only the average speed on a road, but also differences in speed influence road safety (Aarts & Van Schagen, 2006). Firstly, roads with a large speed variance (i.e. large differences between the speeds of vehicles in for instance a 24-hour period) generally are less safe than roads with a small speed variance. Secondly, speed differences between vehicles at the same time and place are related with higher risks. Thirdly, vehicles with a higher speed than average on that road have a higher risk of a crash; vehicles driving slower than average have no higher or lower risk. 

How many crashes are caused by (too) high speeds?

Internationally, it is generally assumed that about a third of the fatal crashes is (partly) due to speeding or improper speeds (e.g. OECD/ECMT, 2006). However, it is difficult to determine precisely when speeding is the main cause, because often other factors besides speed also play a role in the occurrence of a crash. It is particularly difficult to determine objectively when a speed is too high for the conditions. The police therefore do not record speed as the crash cause very often. In the Dutch police crash registration (BRON) this is the case for around 4% of fatal crashes and for less than 2% of the crashes with inpatients.[i]


[i] Note: an inpatient in the BRON database is not the same as what is presently defined as a serious road injury in the Netherlands (hospital admission with a minimum injury severity of MAIS2+ (see SWOV Fact sheet Serious road injuries).

 

Is speeding equally dangerous at all locations?

No, speeding is not equally dangerous everywhere. It is however true for all locations that the number of crashes and their severity increase if the average speed on a road goes up (and the other conditions are unchanged) and that the number of crashes and their severity decrease if the average speed on a road goes down (Aarts & Van Schagen, 2006). 

The effect of a speed increase or reduction on rural roads is proportionally larger than on urban roads (see also Table 1). Furthermore, it is important to realize that the exact relation between crashes and speed on a specific road depends on many factors, including infrastructural characteristics, traffic volume and traffic composition.

Is speeding equally dangerous for all road users?

No, the same (impact) speed does not have the same consequences for all road users. The effects depend mainly on the mass of the vehicles concerned and the degree of protection and vulnerability of the road users involved.

Mass

In the event of a collision, the difference in mass determines which vehicle absorbs which part of the energy released. Occupants of the lighter vehicle are significantly worse off than occupants of the heavier vehicle. Mass differences are evident when we look at trucks and passenger cars, but there are also large differences in mass within the category passenger cars. The difference in mass between a large SUV and a small city car can easily be as much as a factor of 3.

Protection

The mass differences in a collision between a motor vehicle and a unprotected and therefore vulnerable cyclist or pedestrian are of an entirely different order. In this case the masses differ from a factor of 10 (light cars) to almost 700 (when trucks of 50 tons). The survival of vulnerable road users drops dramatically with the rising of the (impact) speed (Figure 1). Recent studies (in Rosén et al., 2011) show that more than 95% of the pedestrians survive a collision with a passenger car at an impact speed of 30 km/h; about 85% of the pedestrians survive at an impact speed of 50 km/h; about 40% survive at an impact speed of 80 km/h; and at an impact speed of 100 km/h only a few pedestrians survive.

Figure 1. The relation between impact speed and fatality rate for pedestrians in a crash with a passenger car according to some recent studies (in: Rosén et al., 2011).

 

Vulnerability

Older road users are physically more vulnerable than young road users. Their chance of surviving a crash at a similar collision speed is therefore considerably smaller (see Figure 2; Davis, 2001; in Rosén et al., 2011).

Figure 2. The relation between collision speed and fatality rate for pedestrians in various age groups in crashes with passenger cars (Davis, 2001; in Rosén et al., 2011).
What speed limits are used in the Netherlands?

In the Netherlands, for urban and rural roads the general speed limit is 50 and 80 km/h respectively. In the late 1990s, many residential areas with a limit of 30 km/h and rural areas with a 60 km/h limit were realized. The roads in these areas are roads where fast traffic has to mix with vulnerable road users. Therefore a low(er) speed limit is set to ensure that this happens relatively safely. The general limit on Dutch motorways is 130 km/h with local or time-dependent exceptions of 80, 100 and 120 km/h. Some rural main roads have a 100 km/h limit; these are roads with a supra-regional or national connecting function. Other limits are also found (e.g. 70 km/h and 90 km/h), but these are much less frequent.

How is the speed limit determined in the Netherlands?

There is no laid down system in Netherlands to determine the speed limit on a road or road section. The function of a road and the location in urban or rural areas are the most important factors. In addition, particularly on motorways, a combination of accessibility, flow, the environment and safety play a role. When only road safety would be the starting point, the limits would be significantly lower (Wegman & Aarts, 2006). Table 2 shows the safe speeds for a number of road types and potential conflicts, where 'safe' means that 90% of the collisions that would occur at that speed, would have no serious injury.

Table 2. Safe speeds for some road types and potential conflicts (adapted from Tingvall & Haworth, 1999).

 

Why do drivers speed?

Many drivers regularly exceed the speed limit. When asked (Figure 3; Duijm et al., 2012) Dutch drivers themselves report doing this because they want to adapt to other traffic, because they are in a hurry, because it's fun, or because they did not notice that they were speeding.

Reasons to keep to the speed limit (Figure 4) are above all road safety, the fact that the limit is mandatory, and the risk of a fine. The environment and fuel costs are clearly less important reasons.

Figure 3. Percentages of Dutch drivers who in 2011 gave these reasons for driving faster than the official speed limit, by road type (Duijm et al., 2012).

 

Figure 4. Percentages of Dutch drivers who in 2011 gave these reasons for keeping to the official speed limit, by road type (Duijm et al., 2012).

 

There are four situations in which drivers may be inclined to unintentionally drive faster than the limit (see also the archived SWOV Fact sheet Speed choice: the influence of man, vehicle, and road):  

  1. After having driven at high speed for a long time, for example on the motorway, road users increasingly underestimate their own speed and unintentionally drive at increasing speed.
  2. At a transition from relatively high speed to a significantly lower speed, road users often reduce speed less than is necessary. This is the case, for example, when leaving a motorway, when entering an urban area, or when a long stretch of straight road is followed by a series of bends.
  3. When there is little peripheral information, for example at night, in case of fog, but also on very 'open' roads in flat rural areas road users miss a reference point to which they can relate their own speed.
  4. When road users drive in a ‘high car’, i.e. when the driver's position is considerably higher than the road surface, such as in an SUV or any other jeep-like car, their perception of speed is distorted the speed seems to be lower than in reality.

More in general, the driving comfort has clearly increased during the last decades. The noise and vibration in the car at high speeds have diminished considerably. This applies not only to the larger and heavier passenger cars, but also to smaller passenger cars. This gives the driver less physical feedback when he is driving at high speeds.

What measures for speed management are there?

Speed management consists of a combination of measures in a logical order (Wegman & Aarts, 2006; Van Schagen & Feypell, 2011):

Step 1: Determine the safe speed limit

What speed is safe depends on the function of the road and – consequently – the composition of the traffic. When motorized traffic mixes with pedestrians and cyclists, the speed must be low (see Table 2). Also the possibility of certain conflicts, such as a lateral or a head-on conflict, influences the safe speed.

 

Step 2: Make sure the limit is credible

Credible means that the limit is consistent with the expectations evoked by the road layout so that drivers are more inclined to keep to the limit (Van Schagen, Wegman & Roszbach, 2004; archived SWOV Fact sheet Towards credible speed limits). Features that invite speeding include roads with long straight road sections, wide roads, roads with a smooth surface, and an open, unobstructed road environment (Aarts et al., 2009). Explanation of why a limit does not suit the road image can also be helpful in increasing the credibility (for example, 'noise', 'school area').

 

Step 3: Give good information about the local limit

In practice it is not always clear which limit applies at a certain location. Information on the local limit is usually given by road signs. However, in the Netherlands, general limits are not indicated with road signs: that is knowledge the road user is expected to have. Sometimes the local speed limit is also shown on the hectometre posts. Increasingly, the limit can also be seen inside the vehicle, often linked with a navigation system.

 

Step 4: Support the limit with speed inhibitors

Where necessary (e.g. near schools, pedestrian and bicycle crossings, single-level intersections) physical speed inhibitors help drivers to choose a lower speed: speed humps, road narrowings, plateaus or roundabouts. It is important that the speed inhibitors are on a logical location and that the dimensions comply with the applicable speed regime, with the desired speed, and with the traffic composition (e.g. CROW, 2014).

 

Step 5: Police enforcement

With the above measures we may assume that many of the speeding offences are prevented. But because drivers can ultimately choose their own speed, offences will always be committed. Therefore focused police enforcement remains necessary for the time being, aimed at both general deterrence and prevention. See also SWOV Fact sheets Police enforcement and Speed cameras: how they work and what effect they have (archived).

 

Step 6: Education and information

Education and information offer support for each of the above measures. They can be used to explain speed measures, such as police enforcement and speed humps, and make people aware of the risks of (too) fast driving. They focus mainly on awareness of the problem and the acceptance of measures. It appears very difficult to influence speed behaviour with education and information (Van Schagen et al., 2016).

 

How effective are speed measures?

Structural improvement of speed behaviour requires a combination of measures (see also under What measures for speed control are there?): safe and credible limits and information about the local limit, if necessary supported by physical speed reduction measures and police enforcement. All this should be explained through education and public information.

The effect of individual measures is as follows:

Speed limit:
A rule of thumb is that the effect of a limit change on the average speed is about a quarter of the size of that limit change (Wilmot & Khanal, 1999; see also Elvik, 2012).

Credible limit:
As the limit is more credible, the average driving speed is closer to the limit and the limit is less often exceeded. Also, there are indications that speed differences between drivers are then smaller (Van Nes et al., 2007).

Speed inhibitors:
Physical measures to lower the speed on road sections and intersections (speed humps, plateaus, roundabouts, etc.) are very effective, provided that a right dimensions are used. In 30 km/h-zones the average speed reduction was 15%, but could be as high as 27% (Vis, 1991).

Police enforcement:
If the number of speed checks in the Netherlands were to be doubled especially on the secondary road network, this would save a minimum of 70 road deaths and 1.060 serious road injuries (Aarts et al., 2014).

Education/information:
The effect of education and public information on speed behaviour is very small. It can, however, help to increase the support for and the effect of other measures (Van Schagen et al., 2016).

ISA:
If all vehicles were equipped with a mandatory and intervening ISA variant this would probably prevent 29% of the crashes (Lai et al., 2012).

Can intelligent speed assistance (ISA) be helpful in speed management?

Speeding offences can be countered with intelligent speed assistance (ISA); see archived  SWOV-Fact sheet Intelligent Speed Assistance (ISA). ISA determines the position of a vehicle and compares the speed with the local speed limit. Then the system gives feedback to the driver (informative/advisory) or makes speeding physically impossible (intervening). ISA is potentially very effective. If all vehicles were equipped with a mandatory, intervening ISA variant, 29% of traffic crashes would probably be prevented (Lai et al., 2012).

Other than the voluntary informative or advisory form of navigation systems, large-scale implementation of ISA has so far not been realized. This is not due to the technique; this is basically available. Main reasons that stand in the way of implementing a more stringent form, are uncertainties around the reliability of information on the local speed limit, liability in case of system  errors, and public support (Van der Pas et al., 2012).

 

Publications and sources

Aarts, L.T., Eenink, R.G., Weijermars, W.A.M., Knapper, A. & Schagen, I.N.L.G. van (2014). Soms moet er iets gebeuren voor er iets gebeurt. Verkenning van mogelijkheden om de haalbaarheid van de verkeersveiligheidsdoelstellingen te vergroten. R-2014-37A. SWOV, Den Haag.

Aarts, L., Nes, N. van, Wegman, F.C.M. & Schagen, I.N.L.G. van (2009). Safe Speeds and Credible Speed limits (SaCredSpeed): New vision for decision making on speed management. In: Compendium of papers DVD 88th Annual Meeting of the Transportation Research Board TRB, Washington D.C., January 11-15, 2009,

Aarts, L. & Schagen, I.N.L.G. van (2006). Driving speed and the risk of road crashes; A review. In: Accident Analysis & Prevention, vol. 38, p. 215-224.

CROW (2014). Richtlijn drempels, plateaus en uitritten. CROW, Ede.

Duijm, S., Kraker, J. de, Schalkwijk, M., Boekwijt, L. & Zandvliet, R. (2012). PROV 2011 Periodiek Regionaal Onderzoek Verkeersveiligheid – Bijlagenrapport. Rijkswaterstaat, Dienst Verkeer en Scheepvaart, Delft.

Elvik, R. (2009). The Power Model of the relationship between speed and road safety: update and new analyses. TØI Report 1034/2009. Oslo, Institute of Transport Economics TØI.

Elvik, R. (2012). Speed Limits, enforcement, and health consequences. In: Annual Review of Public Health, Vol. 33, p. 225-238

Elvik, R. (2013). A re-parameterisation of the Power Model of the relationship between the speed of traffic and the number of accidents and accident victims. In: Accident Analysis & Prevention, vol. 50, p. 854-860.

Lai, F., Carsten, O. & Tate, F. (2012). How much benefit does Intelligent Speed Adaptation deliver: An analysis of its potential contribution to safety and environment. In: Accident Analysis & Prevention, Vol. 48, 63-72.

Nes, C.N. van, Schagen, I.N.L.G. van, Houtenbos, M. & Morsink, P.L.J. (2007). De bijdrage van geloofwaardige limieten en ISA aan snelheidsbeheersing; Een rijsimulatorstudie. R-2006-26. SWOV, Leidschendam.

Nilsson, G. (1982). The effects of speed limits on traffic accidents in Sweden. In: Proceedings of the international symposium on the effects of speed limits on traffic accidents and transport energy use, 6-8 October 1981, Dublin. OECD, Paris, p. 1-8.

OECD/ECMT (2006). Speed management. Organisation for Economic Co-operation and Development OECD/European Conference of Ministers of Transport ECMT, Paris.

Pas, J.W.G.M. van der, Marchau, V.A.W.J., Walker, W.E., Wee, G.P. van & Vlassenroot, S.H. (2012). ISA implementation and uncertainty: a literature review and expert elicitation study. In: Accident Analysis & Prevention, vol. 48, p. 83-96.

Rosén, E., Stigson, H. & Sander, U. (2011). Literature review of pedestrian fatality risk as a function of car impact speed. In: Accident Analysis & Prevention, vol. 43, p. 25-33.

Schagen, I. van, Commandeur, J.J.F., Goldenbeld, C. & Stipdonk, H. (2016). Monitoring speed before and during a speed publicity campaign. In: Accident Analysis & Prevention, vol. 97, p. 326-334.

Schagen, I.N.L.G. van, Commandeur, J.J.F., Stipdonk, H.L., Goldenbeld, C. & Kars, V. (2010). Snelheidsmetingen tijdens de voorlichtingscampagne 'Hou je aan de snelheidslimiet'. D-2010-9. SWOV, Leidschendam.

Schagen, I. van & Feypell, V. (2011). Speed and speed management for road safety: an overview of the findings of the OECD Working Group on Speed Management. In: Fit to Drive: Proceedings of the 5th International Traffic Expert Congress, The Hague, April 6th-8th 2011, p. 81-86.

Schagen, I.N.L.G. van, Wegman, F.C.M. & Roszbach, R. (2004). Veilige en geloofwaardige limieten; Een strategische verkenning. R-2004-12. Stichting Wetenschappelijk Onderzoek Verkeersveiligheid SWOV, Leidschendam. 

Tingvall, C. & Haworth, N. (1999). Vision Zero: an ethical approach to safety and mobility. In: Proceedings of the 6th ITE International Conference Road Safety & Traffic Enforcement: Beyond 2000, Melbourne, 6-7 September 1999.

Vis, A.A. (1991). Effecten van inrichting tot 30 km/uur zone in 15 experimentele gebieden: een evaluatiestudie op basis van integratie van resultaten uit verkeerskundige studies, bewonersonderzoeken en een ongevallenanalyse. R-91-81. SWOV, Leidschendam.

Wegman, F. & Aarts, L. (eds.) (2006). Advancing Sustainable Safety; National Road Safety Outlook for 2005-2020. SWOV, Leidschendam.

Wilmot, C.G. & Khanal, M. (1999). Effect of speed limits on speed and safety: a review. In: Transport Reviews, vol. 19, p. 315-329.

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Updated

21 Nov 2016

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