Speed, speed limits and stopping distances

 

 

 

 

 

Key facts

• Breaking the speed limit or travelling too fast for conditions was recorded as a contributory factor in 25% of fatal crashes in 2017 [1];
• More than half of the crashes where 15-19 year olds were killed have speed as a contributing factor [2];
• Drivers with one speeding violation annually are twice as likely to crash as those with none [3];

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 a quarter of fatal crashes in NZ [4]. 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.

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 [5].

Take action: Make the Brake Pledge to stay under speed limits, slow to 30km/h around schools, homes, shops and roadworks, slow right down for bends, brows and bad weather, and speak out for slowing down.

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 distances in our stopping distance illustration are based on a reaction time of 1.5 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 [6].

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 [7].

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 30km/h would stop in time to avoid a child running out five car-lengths (20m) in front. The same vehicle travelling at 50km/h would not be able to stop in time, and would hit the child at 50km/h, as the driver would still be thinking about braking by the time they reach the child. The greater the impact speed, the greater the chance of death. 

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 exchange 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 [8].

Speed limits

Speed limits on local roads in NZ are set by local councils, speed limits on other roads are the responsibility of the NZ Transport Agency. Limits are set in line with government guidance [9].

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.

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. [10]

More information

• Read our advice for drivers.
• Take a look at our speed fact sheet.
• Use our community campaign kit to call for slower speeds in your area.

End notes

[1] Speeding crashes, Ministry of Transport, 2019
[2] Who dies in crashes in which speed is a contributing factor (2015-2017), Ministry of Transport, 2019 
[3] Crash involvement of motor vehicles in relationship to the number and severity of traffic offenses, SWOV, 2013
[4] Speeding crashes, Ministry of Transport, 2019
[5] Crash involvement of motor vehicles in relationship to the number and severity of traffic offenses, SWOV, 2013
[6] Sokolvskji, E., Automobile braking and traction characteristics on the different road surfaces, 2010
[7] 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
[8] Davis, Dr A., Essential evidence: kinetic energy management, Haddon's matrix and road safety, Bristol City Council, 2015
[9] Land Transport Rule: Setting of Speed Limits 2017, Ministry of Transport, 2017
[10] Zero Road Deaths and Serious Injuries: leading a paradigm shift to a safe system, International Transport Forum, 2016

Last updated: February 2019