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Effectiveness of Policies to Reduce Drivers’ Speed

Road Angel

6 Oct 2022

By now, the notion that speeding kills is a cliché that we have all heard to the extent that no sensible person would argue against it.

Indeed, this cliché is supported by research demonstrating a simple reduction of 10km/h as being associated with a 28% decrease in pedestrian-motorist collisions, with a 67% decrease in major and fatal injuries [1]. Speed limits are therefore important road safety measures (see the blog Speed Limit…less for more information). Ensuring that you drive to the speed limits is equally important.

Try as we might, lapses in concentration do unfortunately occur. We can find ourselves inadvertently driving beyond the speed limit. Suddenly, we see a speed camera ahead. It makes us check our speed and slow down just in time before we are caught (if we react fast enough). Otherwise, it’s a simple sequence of events that can quite easily cause us to be penalised if we don’t rectify our speed in time. Fortunately, there are two measures that help prevent such a situation:

1. UK regulation states that speedometers must never underreport a vehicle's speed, while it must never overreport by more than 10% of the actual speed + 6.25mph [2].

This means if you’re travelling at 40mph, your speedometer may read up to 50.25mph, but it can never read less than 40mph. In general, whilst speedometers do not typically display the real speed of the vehicle, they are not as inaccurate to the extent mentioned above. However, it’s still interesting to note that it can legally overreport by such a notable amount.

2. Speed cameras have tolerances for speeding.

Old laws stated that drivers could be driving beyond the speed limit by 10% + 3mph before they would be penalised, but in most parts of the country, this has now been changed to 10% + 2mph. In London alone, this simple policy change has led to a 259% increase in drivers being penalised, with 347,000 drivers being warned they face prosecution between January and June this year compared to 97,000 in the six months preceding the change [3]. Given that approximately 52% of UK drivers are estimated to usually observe the speed limits, the increased rate of drivers being penalised is likely due to genuine error, rather than intentional behaviour; a smaller proportion of drivers are estimated to occasionally (33%) or routinely (14%) exceed speed limits [4].

According to Auto Express, the tolerances for each constabulary are posted below [5]:

The influence of this policy change on road safety is hard to decipher. However, if the logic is reached that 1mph tolerance reduction leads to an average 1mph reduction in speed, previous research [6,7] indicates that the accident rate would fall by approximately 5% in general, although this varies by road type, such as:

  • Urban main roads and residential roads: 6%.

  • Medium speed urban roads and lower speed rural main roads: 4%.

  • Higher speed urban roads and rural single carriageway main roads: 3%.

These percentages may seem small, but if we take the UK road incident figure from 2021 of 127,967 [8], the general rate of 5% would see a road incident reduction by 6,398. This is a reasonable impact. The problem however is that the previous research does not specifically explore how such reductions in speed influence road incidents at higher speed, such as on motorways. Regardless, the 1mph tolerance reduction could potentially be a significant road safety measure if more people are aware of the policy change and it consequently makes them more cautious with their driving.

Policy has thereby changed to covertly nudge drivers to drive slower, but the policy around the penalties themselves are overtly intended to reduce the chances that a driver will speed. For instance, before the 24th April 2017, a driver caught speeding would receive a penalty that ranged from a Fixed Penalty Notice of £100 with three penalty points, to £1000 or £2,500 for motorway offences. Afterwards, the penalty system became more nuanced, divided broadly into three ‘bands’ of speeding. These are:

A general penalty point system appears to convey a road safety benefit, such as in Spain where there was a 10% reduction in the risk of driver-fatal collisions, and 2% of overall collisions, following the introduction of a penalty system [9]. This means that there would have been an estimated 2,559 extra collisions on UK roads in 2021 if a penalty system was not in place. However, exploring this system further implies that it is only licence suspensions that are consistently associated with traffic safety benefits; increasing severity of other punishments seems unrelated to traffic safety [10]. As such, only elements presented in Bands B and C would theoretically influence driver behaviour. Whether the penalties conveyed in Band A would influence driver behaviour is uncertain.

Measuring the true consequences of these policies is hard to achieve as there are various other road safety measures that are being introduced or becoming more sophisticated, such as the electronic systems in the cars themselves or driving assistance technology (lane assist, speed limiters, etc). Regardless, the world of motoring is changing, with a particular focus on improving road safety. It seems likely that the policies outlined above will lead to safer roads, but the magnitude of their influence is a question that has yet to be answered.

Only time will tell what more needs to be done to reduce the risk of road incidents to zero. Whilst more can, and should, be done, we should not overlook the achievements from the past until now, from the driving test itself in the 1930s, to compulsory seat belt use in the 1980s, and to the policies explored in this document now. We are living in times where the roads are the safest that they have ever been. Now, with the exponential growth of technology, and as we race to deepen our understanding of how technology can assist driving performance, the future looks positive for road safety.


[1] Fridman, L., Ling, R., Rothman, L., Cloutier, M. S., Macarthur, C., Hagel, B., & Howard, A. (2020). Effect of reducing the posted speed limit to 30 km per hour on pedestrian motor vehicle collisions in Toronto, Canada-a quasi experimental, pre-post study. BMC public health, 20(1), 1-8.

[2] Road Safety GB. (2018, 31 January). Police chief: penalise all drivers caught speeding. Retrieved from on 27/09/2022.

[3] The Times. (2022, 25 September). Thousands snared after stealth drop in speed ‘limit’ by 1mph. Retrieved from on 27/09/2022.

[4] Stradling, S., Broughton, P., Kinnear, N., O’Dolan, C., Fuller, R., Bates, H., ... & Hannigan, B. (2008). Understanding inappropriate high speed: A quantitative analysis (Vol. 93). Road Safety Research Report.

[5] Auto Express. (2022, 26 September). UK speed camera tolerances: Met police lowers speed limit enforcement threshold. Retrieved from on 27/09/2022.

[6] Finch, D. J., Kompfner, P., Lockwood, C. R., & Maycock, G. (1994). Speed, speed limits and accidents. Project Report 58. Transport Research Laboratory, Crowthorne, UK.

[7] Taylor, M. C., Lynam, D. A., & Baruya, A. (2000). The effects of drivers' speed on the frequency of road accidents (p. 56). Crowthorne: Transport Research Laboratory.

[8] Department for Transport. (2022, 25 May). Reported road casualties Great Britain, provisional results: 2021. Retrieved from on 27/09/2022.

[9] Novoa, A. M., Pérez, K., Santamariña-Rubio, E., Marí-Dell'Olmo, M., Ferrando, J., Peiró, R., ... & Borrell, C. (2010). Impact of the penalty points system on road traffic injuries in Spain: a time–series study. American journal of public health, 100(11), 2220-2227.

[10] Mann, R. E., Vingilis, E. R., Gavin, D., Adlaf, E., & Anglin, L. (1991). Sentence severity and the drinking driver: relationships with traffic safety outcome. Accident Analysis & Prevention, 23(6), 483-491.

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