Driver Knowledge Tests

What is tailgating?

When one vehicle follows another at a distance that is so close that the vehicle behind couldn’t stop if the vehicle in front stops, this is tailgating. It’s the leading cause of nose-to-tail accidents worldwide. Many drivers tailgate so frequently that their conception of the danger of it is reduced to the point where they don’t believe it’s dangerous at all; their behaviour becomes normalised.

In the UK the police are allowed to give tickets for tailgating on the motorway. In New Zealand, the legal minimum distance you should be behind the car in front is 4m for every 10kph (therefore 20m at 50kph), although the recommendation is much greater than this. In NSW there are laws that allow police to prosecute but they are difficult to prove, so not used frequently. Automated technology has its flaws because if you use a static camera it can’t tell what happened before the moment the photo was taken, i.e. you could be driving along keeping a good safety margin between you and the vehicle in front, then another vehicle pulls in front of you, halving the safety margin. You will then need to drop back, but if you don’t drop back quickly enough then the camera might catch you.

The minimum safe distance is remembered by an easy mnemonic: only a fool breaks the two second rule. Saying this sentence should take around 2 seconds at normal speed and you start saying it when the rear of the vehicle in front passes a point on the road ahead of you, and you should have stopped saying it by the time the front of your vehicle reaches that point. Many road safety experts advise that three seconds is far safer because if the vehicle in front stops abruptly, e.g. it hits something large and stationary, a vehicle traveling behind would have no chance of stopping given that a quick reaction time would be 0.5 seconds, and even sports cars would struggle to stop in 1.5 seconds from 100kph. Most people’s reaction time will be a second or more.

Tailgating also has other undesirable effects: it restricts your vision of the road ahead, and also annoys other drivers, so it could lead to road rage.

Some drivers believe that tailgating helps save fuel. At higher speeds there will be some slipstream effect, but you would have to be so close to the vehicle in front that you would be able to see nothing. Various tests have been done and found that the benefit is negligible. If you are in the sweet spot of 15m  or less at 90kph you might save 20% in fuel, however, you would also hit the back of the vehicle in front before you even have time to apply the brakes if the vehicle in front stopped.

Are you being tailgated?

If you are being tailgated, why is this? Are you driving too slow for the conditions? Are you hogging the overtaking lane (right lane) on a dual carriageway or motorway? The law is clear about these situations. You should not drive your vehicle in a manner that causes it to be an obstacle to other vehicles by driving abnormally slowly. ‘Abnormally slowly’ has a huge grey area in terms of law enforcement, but let’s say you are driving your vehicle at 40kph on a 100kph stretch of road and you have no good reason to do this (i.e. you’re not a tractor or a delivery vehicle making frequent stops, etc), then you could be deemed to be holding up traffic unnecessarily. You must also keep left unless you are overtaking, turning right, making a u-turn from the centre of the road, following marked lanes because the left lane is left-turn only, driving a vehicle that requires you to be in the right-hand lane, avoiding an obstruction, or traffic in every lane is congested.

If you are complying with all of the above, then increase your buffer in relation to the vehicle in front otherwise if you have to stop quickly you’ll have the vehicle behind running into you. Make it easy for the person behind you to get past if they want to.

Is tailgating beneficial?

One argument you never read in the media because it would attract a lot of negative comment is whether tailgating is actually beneficial. Given the prevalence of tailgating – i.e. almost every driver does it – it has a fairly low accident rate. Tailgating allows us to fit much more traffic on a stretch of road than would be possible if we didn’t tailgate. In fact, it’s likely that if all drivers stayed 2-3 seconds behind the vehicle in front, rush hour would be longer and more frustrating for all vehicles.

Taking the minimum guideline of two seconds behind the vehicle in front, and assuming every vehicle is four metres long and traveling at 100kph we can calculate how many extra cars we would have by halving the distance.

Not tailgating:

Required distance behind the car in front: 55.56m

Length of the vehicle plus the distance required: 59.56

Number of vehicles per kilometre: 16.8

Tailgating:

Distance behind the car in front: 27.78

Length of vehicle plus the distance required: 31.78

Number of vehicles per kilometre: 31.5

Therefore, just under double the number of vehicles could be accommodated on the same stretch of road at the same speed at 100kph. This is not a justification for tailgating and we are not endorsing it. It is an observation that tailgating does serve a functional purpose in today’s traffic. If all vehicles were fitted with automated braking systems they could travel much more closely together. This will eventually happen, but for the moment we are relying on our own relatively slow reflexes, and a bit of luck.

Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists

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  1. […] Reality vs. guidelines: tailgating is the largest cause of accidents in rush hour. Those bumps that leave you on the side of the road cause huge delays for other motorists who have to drive around you. If you want to know why tailgating might be beneficial, check this article out. […]

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