A safe separation distance

A safe separation distance

Many road traffic accidents are caused by drivers driving too fast and too close to the vehicle in front. It’s vital you keep a safe separation distance but how can you judge it?

A driver needs to be able to judge a safe separation distance at all times, in all kinds of traffic, in all weather and road conditions. It is much safer for you and the people in front, and your passengers. Plus, if you hit the car in front of you, you are considered to blame. You have no choice as to the space left behind you, but you can control the amount of space in front.

Tailgating – what is that?

Driving extremely close to the car in front is called ‘tailgating’, and is particularly dangerous. If you are being tailgated by someone, then gently ease off the gas and allow the space in front of you to increase. You really don’t want to put yourself in a position where you have to brake from being too close to the car in front.

If the tailgater is still close behind and happens to hit you in the rear, then, if you have left plenty of space in front of you, your vehicle will not impact the one in front, possibly preventing a major pile up. It also makes the insurance situation a little less complicated!

What exactly is a safe separation distance?

A safe separation distance is a safety margin or empty road between you and the vehicle in front. Think about what happens as a pedestrian, when you are walking close behind someone on the street, and they stop suddenly, for some reason or other. What happens? You bump into them or take a sideways swerve to avoid bumping into them. However, if there was more than just a couple of feet between you and this person, you would notice him stopping in good time to avoid him safely. This is how it works on the road, too.

If we follow too closely to the vehicle in front, we leave no time and no space for things to change rapidly. Leaving you having to do an emergency stop or dangerous avoidance manoeuvre, in order not to hit the back of his car. And even this is not guaranteed. Too close means trouble waiting to happen.

How far away should you be from the car in front of you?

It is generally a good idea on urban and suburban roads, to give plenty of space in front.

For example, on dry roads, you can leave approximately 1 metre (1 yard) for every one mile per hour, of your speed.

At 30 mph you will be 30 metres away from the guy in front; enough to encompass the suggested overall stopping distance published in The Highway Code.

What are overall stopping distances?

The Highway Code contains a chart showing overall stopping distances. These are distances a car travels over the time it takes for you to bring the vehicle to a full stop.

These distances are for a well-maintained car, with good brakes and tyres, an alert driver, and a dry road, in daylight. You need to leave enough space for this to happen safely.

As The Highway Code suggests, at 30mph your car will take approximately 23 metres to come to a stop.

This is made up of thinking distance (the time it takes for you to activate your brakes, and the distance you have travelled before they start to affect the speed of the car downwards), and braking distance (the time/distance it takes to come to a stop).

stopping distance formula

The distances shown are a general guide. The distance will depend on your attention (thinking distance), the road surface, the weather conditions and the condition of your vehicle at the time.
(Average car length = 4 metres (13 feet)

In general think of more speed = less time, and, less speed = more time.

So, the overall stopping distance at 30 mph (if you leave 1 metre for every 1 mph), will leave you plenty of space to brake and stop should you need to, without impacting the vehicle in front, or having to make a dangerous swerve or lane change, and reduces the chance of a potentially disastrous skid.

Stopping distances depend on:

  • How fast you are travelling
  • Whether you are on a level road, or a hill going up or down, and the steepness of that hill.
  • Weather; is it good and dry, or is it wet or icy?
  • Tyres; are they good tyres and properly inflated, or worn or badly inflated?
  • Brakes, are they working well, are they stopping you in a straight line?
  • Your ability as a driver, are you ill, tired, on medication, have drunk alcohol, are distracted? All of these can affect your reactions when applying brakes.

Separation distances are essential to allow you time to see and react appropriately to any potential or developing hazard.

For an example scenario, just say you are close up behind something a little larger than an ordinary car (perhaps a bus or a commercial vehicle). If you are so close that you cannot see the driver’s side mirrors, then you are invisible to him, as he cannot see you. Plus, if the large vehicle has to stop suddenly then you have no chance of seeing the potential in the road ahead, and could very easily hit the rear of that vehicle. So, stay back even more than normal when travelling behind large vehicles. They can block your view of the road ahead, and reduce your ability to forward plan.

Exceptional circumstances for a safe separation distance

Sometimes it is not practical to allow and keep the separation distance, such as in heavy slow-moving urban traffic, because of the limited road space available.

But busy, slow-moving traffic is the norm these days, and you must keep at least your thinking distance clear – and much more than that if the road is wet or slippery.

The overall stopping distance is really the only safe separation distance. Anything less than this can be considered a risk.

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The two-second rule for a safe separation distance

Some of you might have heard about this, but for those who have not, this is a simple technique for helping to judge separation distances. It is especially useful on faster roads and motorways where speeds are considerably higher than normal.

How to check the two-second rule

The driver of the following car must be at least 2 seconds behind the vehicle in front:

  • The driver is alert
  • The car is in good condition, good tyres, good brakes
  • The weather is dry.

Take note of the vehicle in front when he passes a post or bridge support, and then count 2 seconds. You should not arrive at the same spot before the 2 seconds are up.  If you are too close, then carefully drop back and retest the gap.

If you are tired or driving a less than perfect car, or the weather is bad, then your 2-second rule should be extended to 4 seconds or more.

Always know your limitations, and remember that: “Only a fool breaks the 2-second rule.”

Multiple collisions or pile-ups are caused by driving too close and too fast, which leads to drivers being unable to brake in time. You can avoid this by looking well forward, checking how the traffic is performing, getting clues from large vehicles, looking for buses pulling in and out, taxis stopping and turning, junctions and pedestrians.

Maintain that safe separation distance as much as you can. It is always better to drive defensively, allow yourself enough time for the journey, and arrive alive, but maybe a bit late, than to not arrive at all.


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