What Factors Affect Car Stopping Distance?


Nov 02nd, 2020

What Factors Affect Car Stopping Distance?

Why do you need to know about car stopping distance? Whether you’re studying for a learner’s theory test or you’ve been driving for years now, understanding the concept of stopping distance is crucial. First, it’s a favourite subject for the theory test. Second, its practical application will be useful in appreciating the wisdom of avoiding common driving mistakes, such as tailgating, overspeeding, and drunk driving. 

So, let’s get down to what car stopping distance is, the factors that affect it, and how to get the most from this article to stay safe on the road. We won’t touch on the maths and physics of things, so no worries! This is going to be light reading for you, promise.

What is car stopping distance?

Allow me to illustrate the concept of car stopping distance with an example, so it’s more visual that way. Imagine driving your car peacefully at 50 km/h when, suddenly, you notice a kangaroo on the road, some 45m ahead. Do you think you’d be able to stop in time and avoid a crash? 

Your aim would be to bring your vehicle to a stop in the safest and shortest distance possible. That distance is your car stopping distance, best expressed as:

Stopping Distance = Thinking Distance + Braking Distance

Thinking distance is the distance that your car travels before you react and apply the brakes. The reaction time may vary from 0.2s to 1.5s for different drivers, so the distance travelled within that period will also change depending on the time and initial speed of the car. The figures given here are only averages – a driver may take more or less time to think and react. If your car’s speed is 50 km/h, your average thinking distance will be within 10m-15m. 

Braking distance, on the other hand, is how far your car travels after you have stepped on the brakes. Using equations of motion, we approximate the braking distance to be close to 15m on dry roads and 20m on wet. 

Thus, if you were in the situation described above, the odds are that you'd be able to reduce your speed and stop in time to avoid crashing on the kangaroo or minimise the impact. In our article on how to avoid animal crashes in Australia, we recommend braking in a straight line, which results in maximum braking force and the least braking distance possible. 

Is thinking distance always longer than braking distance?

A road safety page from Queensland’s Department of Transport shows that thinking distance seems to be longer than braking distance when speeds are relatively low (40km/h to 60km/h). However, this trend reverses as speed increases, with the braking distance even doubling the thinking distance at speeds of 100km/h on wet conditions. 

This information is based on studies done in ideal and controlled conditions, but we know that in the real world, several factors may affect the actual figures. 

What factors influence thinking and braking distances?

Thinking Distance

You calculate thinking distance by multiplying the car’s initial speed and the driver’s reaction time, which is affected by:

  • Fatigue or lack of sleep
  • Driver’s vision, age, and driving experience 
  • Distractions, e.g. using a phone or eating
  • Drowsiness or other effects of medications
  • Impaired judgment due to alcohol or drugs

Always remember that thinking distance is in direct proportion to speed and reaction time. 

Braking distance

Your speeding car possesses kinetic energy due to its motion. Applying the brakes generates friction that opposes the vehicle’s movement. The braking force is the work required to reduce the vehicle’s kinetic energy, slow it down, and bring it to a halt. This whole chain of events is affected by the following: 

  • Speed of the car – the faster the vehicle travels, the longer is its braking distance and the greater is the required braking force.
  • Mass of the car – the braking system works harder to stop a heavier vehicle.
  • Presence of safety features – ABS, EBA, AEB, and ESC all aid in an efficient braking system.
  • Road surface/gradient and weather conditions – wet, dry, and icy roads are variables that affect traction.
  • Vehicle conditions – worn brake pads, incorrect tyre pressure, and bald tyres reduce your car’s ability to grip the road. In short, it reduces the friction produced, which is what you need to counter your car’s kinetic energy. 

What are typical stopping distances for a regular car under ordinary conditions?

Let’s see how long cars travelling at different speeds take to stop completely.

Stopping distance = TD + BD

@ 32 km/h:

                   Stopping distance = 6m + 6m = 12m (approx. 3 car lengths)

@ 48 km/h:

                   Stopping distance = 9m + 14m = 23m (approx. 6 car lengths)

@ 64 km/h:

                   Stopping distance = 12m + 24m = 36m (approx. 9 car lengths)

@ 96 km/h:

                   Stopping distance = 18m + 55m = 73m (approx. 18 car lengths)

You may have noticed that the thinking distance changes in direct proportion to speed. Say, when speed was increased by 50% (from 32km/h to 48km/h), the TD also increased by 50% (6m to 9m). The same is true when we doubled the speed (32km/h to 64km/h), the TD also doubled (6m to 12m).

However, the same cannot be said about the braking distance, which changes exponentially with every increase in speed. 

Note: All these figures are approximates with an assumed reaction time of 0.68s.

Can you reduce your car’s stopping distance?

With your knowledge on the factors that affect vehicle stopping distance, there’s no reason that prevents you from reducing your car’s stopping distance. While you may not be able to change some factors, such as the weight of your car, road and weather conditions, and a few others, but there are variables you can control.

The most obvious variable, and one with the most impact to stopping distance, is speed. The faster your car travels, the longer it takes to bring it to a stop.

Don’t drive when you’re not in your best form, such as, when you’re ill, drunk, sleepy, intoxicated, or distracted. 

Keep your car’s braking system healthy at all times. You can do this by replacing worn pads and brake discs, inspecting your tyres regularly, and never skipping servicing schedules. For auto parts that need replacing, always make it a point to buy only from reliable sellers. One way to do this is to request parts through Carpart.com.au’s Auto Parts Finder tool. It’s free to use, so try asking for a quote today! You may also bookmark our site or subscribe to our blog for more auto advice similar to this article.

By Jeannette Salanga (JMSL)