Speed, speed limits and stopping distances

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Key facts

  • Breaking the speed limit or travelling too fast for conditions was recorded (by police at crash scenes) as a contributory factor of 23% of fatal crashes in 2015 [1];
  • Drivers with one speeding violation annually are twice as likely to crash as those with none [2];
  • A Brake and Direct Line survey found that four in 10 (40%) of drivers admitted that they sometimes driver at 30mph in a 20mph zones [3];
  • More than a quarter of drivers surveyed (26%) admitted to ‘regularly’ speeding in areas designed to keep children and other road users safe. [4] 

Introduction

Speed is a critical factor in all road crashes and casualties. Driving is unpredictable and if something unexpected happens on the road ahead – such as a child stepping out from between parked cars – it is a driver’s speed that will determine whether they can stop in time and, if they can’t stop, how hard they will hit.

Reducing and managing traffic speeds is crucial to road safety. Breaking the speed limit or travelling too fast for conditions is recorded (by police at crash scenes) as a contributory factor in almost one in four (23%) fatal crashes in the UK [5]. This is arguably a gross underestimate, because whether or not a vehicle is judged to have been speeding or going too fast for conditions, the fact it was involved in a collision means it was going too fast to have stopped in time. In this way, speed is always a contributory factor, albeit often in combination with other causes: no one was ever killed by a stationary vehicle.

Dutch research has found drivers with one speeding violation annually are twice as likely to crash as those with none, and this increases further for drivers who commit repeated speed violations [6].

makethepledgeTake action: Make the Brake Pledge to stay under speed limits, slow to 20mph by schools, homes and shops, slow right down for bends, brows and bad weather, and speak out for slowing down.
 
GO20researchreportLearn more: Read our major research report on the extent of 20mph limits in Great Britain and the barriers faced by local councils in implementing them.
 
 
go20quizsma  Test your knowledge: Try our GO 20 quiz

Stopping distances

Stopping distances include the distance travelled while the driver notices a hazard and applies the brakes (thinking distance), and while the vehicle comes to a full stop from its initial speed (braking distance). The government's official estimates of stopping distances for cars are shown below. [7]

stopping-distances

Source: Department for Transport, 2007

The distances above are based on a reaction time of 0.67 seconds, which assumes the driver is alert, concentrating and not impaired. Driving when tired, distracted or impaired significantly increases reaction times, so the thinking distances above should be regarded as minimums.

The braking distance depends on how fast the vehicle was travelling before the brakes were applied, and is proportional to the square of the initial speed. That means even small increases in speed mean significantly longer braking distances. Braking distances are much longer for larger and heavier vehicles, and in wet or icy conditions, so again these figures are a minimum [8].

Technology such as anti-lock brakes and stability control are designed to enable greater control over the vehicle, not shorten stopping distances. There may be a very small reduction in braking distance with modern technology, but not enough to significantly affect your overall stopping distance [9].

Whatever technology a vehicle has, the basic fact remains that the faster you drive, the longer your stopping distance, and therefore the less chance you have of stopping in time in an emergency.

Learn more: Read our advice for drivers on staying slow and safe.

Impact speed

Driving faster not only lessens drivers’ chances of being able to stop in time to avoid hitting someone or something. It also means if they can’t stop in time, they will hit with greater impact. The greater the impact, the greater the chances of causing serious injury or death.

A vehicle travelling at 20mph (32km/h) would stop in time to avoid a child running out three car-lengths in front. The same vehicle travelling at 25mph (40km/h) would not be able to stop in time, and would hit the child at 18mph (29km/h). This is roughly the same impact as a child falling from an upstairs window. The diagram below illustrates the impact at various speeds. The greater the impact speed, the greater the chance of death. A pedestrian hit at 30mph has a very significant one in five chance of being killed. This rises significantly to a one in three chance if they are hit at 35mph [10].

Roof fall distances

Speed and mass are the properties of energy exchanged in a road collision in the form of kinetic energy, the level of energy exchanged has a significant impact on the severity of the crash. It is believed that the exchanged of energy can be calculated equal to half the vehicle’s mass times the vehicle speed squared; which means that even smaller increases in speed can lead to an increase in impact severity [11].

Speed limits

Speed limits on local roads in the UK are set by local authorities according to government guidance that they are “evidence-led” and aimed at developing a road environment which is safer and fit for purpose [12].

Speed limits on all other UK roads follow national standards. Speed limits are limits, not targets – they are set as the top speed for any particular road, and it is frequently safer to travel at much lower speeds, such as in bad weather, poor visibility, and where there are (or could potentially be) people on foot and bicycle, especially children.

Unfortunately, many drivers do not always obey speed limits. A Brake and Direct Line survey found that four in 10 (40%) of drivers admitted that they sometimes driver at 30mph in a 20mph zones. More than a quarter of drivers surveyed (26%) admitted to ‘regularly’ speeding in areas designed to keep children and other road users safe. [13]

Effective speed management is considered central to a ‘safe system’ approach to road safety, crucial to reducing casualties and enabling walking and cycling. The safe system principle acknowledges that people can make mistakes behind the wheel and that there are known limits to ‘the capacity of the human body to absorb kinetic energy before harm occurs’. Within a safe system, effective speed management works holistically with vehicle design, road infrastructure and road user behaviour, to produce an overall safety effect greater than the sum of its parts. [14]

Learn more: Read our fact pages on speed limits in communitiessafe speeds on country roads, and motorway speeds.

Take action: Support Brake’s GO 20 campaign for slower speeds in towns, cities and villages, and Brake’s rural roads not racetracks campaign for slower speeds on country roads.

More information


End notes

[1] Reported Road Casualties Great Britain: Annual Report 2015, Department for Transport, 2016, table RAS50008
[2] Crash involvement of motor vehicles in relationship to the number and severity of traffic offenses, SWOV, 2013
[3] Report on safe driving: speed, Brake and Direct Line, 2016
[4] Ibid
[5] Reported Road Casualties Great Britain: Annual Report 2015, Department for Transport, 2016, table RAS50008
[6] Crash involvement of motor vehicles in relationship to the number and severity of traffic offenses, SWOV, 2013
[7] The Highway Code: rule 126, Department for Transport, updated 2016
[8] Sokolvskji, E., Automobile braking and traction characteristics on the different road surfaces, 2010
[9] Benefit and Feasibility of a Range of New Technologies and Unregulated Measures in the fields of Vehicle Occupant Safety and Protection of Vulnerable Road Users, TRL, 2015
[10] Impact Speed and a Pedestrian’s Risk of Severe Injury or Death, the AAA, 2011
[11] Davis, Dr A., Essential evidence: kinetic energy management, Haddon’s matrix and road safety, Bristol City Council, 2015
[12] Working together to build a safer road system: road safety statement, Department for Transport, 2015
[13] Report on safe driving: speed, Brake and Direct Line, 2016
[14] Zero Road Deaths and Serious Injuries: leading a paradigm shift to a safe system, International Transport Forum, 2016


 Page last updated: November 2016