Understanding stopping distances and how they’re calculated is important for everyone, from those about to take their theory and hazard perception test to those who drive every day and have done so for many years. Stopping distances also come into play when you have black box insurance: careful driving and coming to a stop within plenty of time will help to improve your driving score, and in turn could see you enjoy money back on your policy throughout the year.
We’ve looked further into stopping distances, how to calculate them and why they’re so important in this in-depth guide. Use this as a reference when wrapping your head around stopping distances, to make you a safer driver and to smash that theory test if it’s on the horizon.
Why you need to understand stopping distances
1. It can make you a safer driver - Calm and controlled braking and ensuring stopping distances are safely achieved makes you a safer driver, in all weathers and driving conditions. Important for yours and other road users’ safety.
2. It will make your theory test a little easier - If you’re studying for your theory test, understanding how to calculate stopping distances and what impacts them will ensure you get those important questions correct.
3. It can save you money - At WiseDriving, we monitor how sharply you brake and slow down when out on the road, via a black box device installed in your car. We calculate a driving score, based on how you drive, and this determines how much you pay for your car insurance.
Safer driving = a better score, which means you pay less! If you don’t keep stopping distances in mind and regularly brake sharply instead of coming to a safe and controlled stop, then your driving score could be negatively affected and this could potentially increase how much you pay for insurance.
You’ll also save money on general wear and tear on your car. Coming to a safe stop means less impact on your brakes and your tyres, helping them last longer and require less maintenance. It’s also better for the environment, as you drive slower and come to a stop in plenty of time you will reduce your overall emissions and save fuel too.
Want to know more about stopping distances and how to ensure you’re sticking to them, to get the very best driving score and price for your car insurance? Read on...
So, how do you calculate stopping distances?
It’s worth refreshing yourself on how stopping distances are calculated, no matter how long you’ve been driving to enjoy some of the benefits mentioned above. There’s an easy to remember calculation for this:
thinking distance + braking distance = stopping distance
What is thinking distance?
Thinking distance is how far the car travels once the driver realises there is a hazard and thinks about applying the brakes. Thinking distance is generally 1 foot for every mph you are travelling, so the thinking distance for a car travelling at 30 mph is 30 ft.
What is braking distance?
To calculate your braking distance, you need to times your thinking distance dependent on the speed you are travelling at. If you're travelling at 20 mph, start from 2, and then add on 0.5 for every 10 mph above 20 you are doing. So if you are travelling at 30 mph, your overall stopping distance would be 75 ft (Thinking distance 30 ft x 2.5 = 75).
Here are the formulas, broken down, for the most common road speed limits:
Some people prefer to think about stopping distances in terms of car lengths. The average car is around 15 ft long so this means the stopping distance for someone travelling at 40 mph would be around 8 cars (120 divided 15). This is why it’s so important to leave a reasonable gap when driving behind other vehicles and braking gradually, in plenty of time, to come to a safe stop.
It’s worth noting that these estimated stopping distances are based on drivers that are not impaired by any distractions, driving a vehicle that is in good condition and in dry weather. Here, we’ve taken a look at what can affect stopping distances to ensure you’re prepared for future driving.
What can affect stopping distances?
- Weather and road conditions
- Drugs and alcohol
- Driver distractions
- Tyre condition
- Weight of the car
- The incline of the road
The speed you are driving at
How fast you are driving will impact the overall stopping distances. A faster speed means you’ll take longer to come to a stop and so you should always leave plenty of time to brake when coming off motorways or dual carriageways, which tend to be higher speed. Also, if you have telematics insurance you are likely monitoring your speed to ensure you can enjoy money back throughout the year, as you drive to the speed limit and safely.
The weather that day
The weather has a real effect on stopping distances, wet weather in particular. Driving in the rain can double your stopping distance, which means you need to leave even more time to brake, while ice and snow can increase your stopping distance by up to ten times! This is because the car has less grip on the road and is why it’s recommended that you drive at a slower speed in these conditions.
How tired the driver is
Motorway and late night driving can lead to drowsiness behind the wheel, which can greatly impact stopping distances and even lead to accidents. Medication can also have this effect, so ensure you check the label before you take medication and drive. If you ever feel tired when driving find a safe place to stop, consider having a coffee or some sugar to boost your energy levels and don’t set out again until you feel awake and clear.
Driving under the influence of drugs or alcohol
It’s not only illegal to drive under the influence but also very dangerous for you and other road users. Drugs and alcohol can slow down your reaction time, increasing your overall stopping distance as you react later than you should.
Distractions in the car
There are lots of things that can distract while behind the wheel, we’re only human, but it’s so important you try to remove or avoid these when driving. Here are a few examples of distractions that can increase your stopping distance, as you react later than you perhaps normally would:
- Mobile phones - Using your phone is illegal while driving. Even a hands-free call can cause a distraction and if you’re using the phone for directions this will take away some of your attention from the road.
- Radio - Tuning to a new station or changing a CD can cause a distraction and in turn impact stopping distances.
- Heating - Playing around with the heat and ventilation takes your attention away from what’s happening on the road.
- Children in the car - Kids can cause distractions in the back seat, whether they’re fighting, crying or have had an accident. Try to pull over if the kids are causing a distraction and resolve this before driving again.
- Sat navs - Great for getting you from A to B but distracting if you need to look at the map or have taken a wrong turn.
- Pets in the car - Whether you’re off to the vets or taking them somewhere for a walk, a pet can be a major distraction. Keep them safe by placing them in a comfortable carry crate, strapped in with a pet-friendly harness or in the boot with an appropriate cage installed to ensure they can’t climb over onto the backseat.
- Passengers - People can be a major distraction if they take away your attention from the road. Make sure your passengers aren’t distracting you, even if it means asking them to be a little quieter.
The condition of your brake pads
Older, worn brake pads are not as effective and so this means stopping distances could be increased, resulting in a higher risk of an accident and dangerous driving. Ensure your brake pads are changed when recommended to not only improve your driving experience but ensure you are safe on the road and good stopping distances are maintained. ABS brakes can help you remain in control when steering while braking but won’t necessarily improve your stopping distance.
The condition of your tyres
Tyre tread has a lot to do with stopping distances and, according to research conducted by RoSPA, tyre treads under 3mm increase stopping distance significantly. Of course, the legal minimum tread depth is 1.6mm, so ensure you check yours regularly.
The tyre pressure can also have an effect too, with under or over-inflated tyres resulting in less rubber on the road, so less grip when braking - this will increase stopping distances.
The weight of the car
The heavier the car, the longer the stopping distance so ensure you take care when loading your vehicle up with heavy items or when you’re offering a lift to two or more friends. You’ll need to allow for more distance between yourself and other cars on the road and brake earlier before roundabouts and junctions.
The road's incline
Unsurprisingly, you’ll need to apply the brakes sooner when travelling downhill as you gain speed and momentum. Travelling downhill will increase your overall stopping distance, while level ground and no other road conditions will result in a normal distance.
Stopping distances and impact force
Speed is the main catalyst for a longer stopping distance and with greater speed comes greater impact if you were to have an accident or hit something.
This is why school zones implement a 20 mph restriction on drivers: a car travelling at 20 mph is more likely to stop in time to avoid hitting a pedestrian three car lengths ahead. However, a car driving at 25 mph would not.
The force of hitting something at 25 mph equates to the impact of falling out of a first-floor window. This just confirms why it’s so important to stick to restricted speed limits - they’re there to protect you and others around you.
Driving and the two-second rule
While you may need to learn the formulas for stopping distances to improve your driving and your driving score if you have telematics insurance, working them out in your head isn’t that easy while on the road.
This is why many people adopt the two-second rule to ensure they always leave enough distance between them and the car in front and plenty of time to stop.
What is the two-second rule?
The two-second rule should only be used in dry conditions. It means if you have to brake sharply there is enough time for you stop without hitting the car in front and a car behind you also has enough warning.
Here is how to ensure you’re driving by this rule:
When driving behind another vehicle, choose a marker in the distance (a lamp post or tree maybe) and once that vehicle has passed that marker count to two. If you reach it in under two seconds that means you’re driving too close and should hold back to leave a safe distance.
In wet conditions, you should double the rule to four seconds as water on the road surface means less grip and a longer stopping distance.
Ready to test your knowledge on stopping distances? Click here and take our quiz to see how much you really know.