Understanding stopping distances and how they’re calculated is important for everyone. Stopping distances also come into play when you have black box insurance. Careful driving and coming to a complete stop within plenty of time will help to improve your driving score. In turn, it could also 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. The aim is to make you a safer driver and help you 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 - In all weathers and driving conditions, calm and controlled braking is vital. It helps to ensure stopping distances are safely achieved, making you a safer driver. Important for your safety and that of other road users.
2. It will make your theory test a little easier - 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 may save you money - WiseDriving install black boxes in cars. They help to monitor how sharply you brake and slow down when out on the road. 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 regularly brake sharply, then your driving score could be negatively affected. 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? Our guide aims to ensure you’re sticking to them. Doing so helps to get the very best driving score and price for your car insurance. So, let’s find out more...
So, how do you calculate stopping distances?
No matter how long you’ve been driving, it’s worth refreshing yourself on how stopping distances are calculated. This may help you to enjoy some of the benefits mentioned above. There’s an easy to remember calculation for stopping distance:
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:
The average car is around 15 ft long. This means the stopping distance for someone travelling at 40 mph would be around 8 cars (120 divided by 15). This is why it’s so important to leave a reasonable gap when driving behind other vehicles. You should always brake 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:
- driving a vehicle that is in good condition
- driving in dry weather
- not impaired by any distractions
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?
- Speed
- Weather and road conditions
- Tiredness
- Drugs and alcohol
- Driver distractions
- Brakes
- 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. You should always leave plenty of time to brake when coming off motorways or dual carriageways, which tend to be higher speed.
If you have telematics insurance, you are likely monitoring your speed. This will help to ensure you can enjoy money back throughout the year, as you drive to the national speed limits 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.
Ice and snow can increase your stopping distance by up to ten times! This is because the car has less grip on the road. For that reason, 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. This 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. You should also consider having a coffee or some sugar to boost your energy levels. Don’t set out again until you feel awake and clear.
Driving under the influence of drugs or alcohol
It’s illegal to drive under the influence, and it’s also very dangerous for you and other road users.
Drugs and alcohol can slow down your reaction time. This increases your overall stopping distance as you react later than you should.
Distractions in the car
There are lots of things that can be distracting while we’re 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:
- Mobile phones - Using your phone is illegal while driving. Even a hands-free call can cause a distraction. 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 song 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. They might be 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.
- 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.
- 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
- strapping them in with a pet-friendly harness
- placing them in the boot with an appropriate cage installed to ensure they can't climb over onto the backseat
The condition of your brake pads
Older, worn brake pads are not as effective. 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. This will not only improve your driving, but also ensure you are safe on the road with good stopping distances maintained.
ABS brakes can help you remain in control while braking, but they won’t necessarily improve your stopping distance.
The condition of your tyres
Tyre tread has a lot to do with stopping distances.
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. Under or over-inflated tyres put less rubber on the road. This gives less grip when braking and will increase stopping distances.
The weight of the car
The heavier the car, the longer the stopping distance. Make sure you take care when loading your vehicle with heavy items or when you’re offering a lift to two or more friends. You’ll need to leave more space between yourself and other cars 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. 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.
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
Learning the formulas for stopping distances can help to improve your driving and your driving score if you have telematics insurance. That said, 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. If you have to brake sharply, the two-second rule means there is enough time for you stop without hitting the car in front. It also gives a car behind you 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.
Correct at the time of publication. Information in this article has been gathered from various sources, therefore WiseDriving cannot be held responsible for any view or opinion expressed herein.
