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10 things that can affect your stopping distance

Kwik Fit | Wednesday 18th May 2016 12:02pm

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At some time or another most drivers will have to perform an emergency braking maneuver, be it due to a hazard in the road, or the car in front also stopping sharply. When it happens you’re going to want the car to stop in the shortest possible distance to avoid an accident. Here are 10 factors that can have an effect on how fast your car can stop; what measures you can take to reduce your stopping distance and what actions to avoid that could extend it. 

1. Speed

Your stopping distance is actually made up of two factors – thinking distance and braking distance. Thinking distance is the time it takes for the driver to process the information and react, from seeing an obstacle to pressing the brake pedal; whereas braking distance is the length travelled from pressing the brake pedal to the car coming to a complete stop. 

Your speed is one of the only factors that has an effect on both your thinking distance and braking distance. Put simply, the faster you are going, the greater the distance travelled before you apply the brakes (thinking distance) and the vehicle comes to a complete stop (braking distance). Between 20-40mph,  your vehicle takes an average of 12 additional metres, or three car lengths, to come to a complete stop for every 10mph you are travelling, that’s why it is so crucial to observe the local speed limit and slow down particularly in residential areas. Over 40mph, this number increases still further and a car travelling at 70mph on a motorway will take an average of 96 metres or 24 car lengths to come to a stop so be sure to keep your distance from the car in front.

2. Brakes

ABS has become commonplace in modern vehicles but does not actually help a great deal in terms of reducing your stopping distance. Rather, anti-lock brakes help the driver to maintain control of the vehicle in an emergency brake situation. However, properly maintained brakes can make a big difference.

Brake pads have a block of friction material that pushes against the brake disc when the brakes are applied. This friction material wears down over time and the brake disc can become grooved causing them to overheat and lose stopping power. Therefore, well maintained brakes will ultimately reduce your stopping distance.  Brakes can also be affected by wet roads and standing water leading to moisture between the pads & discs that can make them less effective at bringing your vehicle to a stop. If you have driven through deep water, make sure you pump the brake pedal a few times while driving slowly to dry them out and ensure they work when you need them.

3. Tyre Pressure

Tyres need to maximise their contact with the road in order to provide the best possible stopping distance. When tyres are over or under inflated though, the tread contact patch is reduced. Underinflated tyres will make more contact with the road on the outer edges of the tyre whereas overinflated tyres make more contact in the centre. Both are bad news for your tyres and stopping distance. Not only does this cause irregular wear of the tyres, but traction will also be reduced meaning your tyres are less effective at biting into the road surface and bringing the car to a halt. Checking your tyre pressure every month and using the recommended pressure in your vehicle hand book is an easy way to maintain your tyres and their ability to stop the vehicle.

4. Tyre Wear

By law, your tyres need to be changed when the tread depth reaches 1.6mm. However, the remaining tread depth on your tyres can have a massive difference on your braking distance long before you reach this absolute minimum. Most tyres begin life with around 8mm of tyre tread which gradually wears away the more you use them (other factors such as extreme braking can also accelerate the rate of wear). As the tread reduces, so too does the tyre’s ability to grip the road. At 30mph on a wet road, a car with brand new tyres with 8mm of tread will come to a stop in 25.9 metres. The same car travelling in the same conditions but fitted with tyres with just 3mm of tread remaining would come to a stop in 35 metres. That’s 35% further despite the tyres still being perfectly legal. When the tyres reach the minimum of 1.6mm of tread, the stopping distance increases to 43 metres, that’s almost double the stopping distance of the new tyres!  

5. Tyre Quality

Buying premium tyres from known manufacturers such as Michelin, Goodyear or Pirelli provides peace of mind that you are buying a quality tyre product. But countless tyre tests show that premium tyres really are worth the extra cash when it comes to control, grip and stopping distance, and consistently outperforming their budget counterparts. When travelling at 60mph a car fitted with premium tyres could stop as much as 16 metres shorter than a set of budget tyres despite both sets of tyres having a full 8mm of tread. Premium tyres have other proven benefits including increased fuel efficiency, lifespan and aquaplaning resistance.

6. Road Conditions

While there are measures you can take to shorten your braking distance like ensuring your tyres are in good shape, the weather is something that we unfortunately have no control over. Yet road conditions like standing water, ice and snow can have a huge impact on your stopping distance. Adding any slippery surface that reduces the friction between your tyres and the road is inevitably going to have an effect on your braking. In heavy rain aquaplaning can occur where the tyres cannot disperse the water between the tread and the road surface quickly enough leading to a loss of control. In wintery conditions snow can become compacted in the tyre tread which greatly reduces the effectiveness of the tyres and their grip on the road. This can lead to sliding and stopping distances 10 times greater than on a dry road.

The best defence when driving in bad weather is to keep your distance, take it slow and make sure you can maintain a good… 

7. View of the Road

Visibility is one of a number of factors that do not affect your braking distance per se but can inhibit your thinking distance. The longer it takes for you to spot hazards in the road, the more time will have passed before you hit the brake pedal. A dirty windscreen will reduce your view of the road so make sure you top up your screen wash regularly and make sure your wipers are working properly. Damaged wiper blades can actually reduce your visibility even further by smearing dirt across your field of vision so make sure you replace any damaged parts immediately. On cold, frosty mornings don’t be tempted to set off before the windscreen has completely cleared. Get out there a few minutes early to warm the car up and scrape all the ice off to make sure you get to work safely and on time.

8. Distractions

Keeping your eyes on the road at all times will help you spot hazards and reduce your thinking time, but it’s easy to become distracted, especially in this digital age of gadgets and in-car tech. Mobile phones are the biggest problem when it comes to driver concentration and you should avoid using your phone at all when driving unless you have bluetooth connectivity allowing you to make and receive calls without looking at the phone. Programming the sat nav and playing with the radio can also be big distractions but motor manufacturers are working hard to integrate these systems into their vehicles using voice commands and other inputs that keep the driver’s attention on the road.

9. Drink/Drug Driving

It should go without saying that you shouldn’t drink and drive following decades of road awareness campaigns highlighting the dangers and consequences of drink driving. But more recently there has been an increased focus on ‘drug driving’ and since March 2015 it has also been an offence to drive under the influence of certain drugs with penalties including a minimum 12-month driving ban, an unlimited fine and up to six months in prison.

Alcohol and drugs including cannabis and cocaine increases the time it takes to process information. A driver who is under the influence of drink or drugs could take a few extra vital seconds to spot a hazard such as a pedestrian crossing the road and apply the brakes. There is also evidence to suggest that a drink/drug driver would not press the brake pedal as hard in an emergency stop situation because their senses are impaired.  

10. Tiredness

As many as one fifth of accidents on monotonous roads like motorways may be caused by drivers falling asleep at the wheel. But even if you don’t drop off, driving while tired can severely slow your reaction time and impair your decision making ability. If you notice yourself getting tired or losing concentration while driving, make sure you find somewhere safe to stop and take a break. Take a 15 minute break every two hours on long journeys and share driving duties when possible. This will give you the best chance of staying alert and optimising your stopping distance.


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