Following Distances and the Conditions

[Astraist]

A trainee asked me a simple question, to which I was surprisingly left without a good answer: Why does the following distances need to be increased in deteriorated road conditions, like when the road is damp, where the traditional rule maintains that a four second gap should be adopted?

In theory, the more slippery the surface, the less stopping force be applied by the first car during the first second of braking, when we as the following driver still haven't reacted, so hypothetically the gap could be decreased on a wet road, rather than be increased.

I did came up with some answers, but I found myself unsatisfied with them somehow:

1. Increase gap to make allowances for the differences between different vehicles (and their maintenance and drivers) since the differences in stopping distances sharpen as the conditions deteriorate

2. Make allowances for the following driver to avoid being rear-ended

3. Make allowances and increasing vision based on the increased likelihood of the driver ahead getting into a collision that will halt his/her progress suddenly and turn them into an effectively static obstacle

4. Reduce resistance to avoid mild braking efforts (when the driver in front slows down or stops) that, in slippery conditions, might cause the car to slide.


Any thoughts?

[michael769]

1. Increase gap to make allowances for the differences between different vehicles (and their maintenance and drivers) since the differences in stopping distances sharpen as the conditions deteriorate

I think this is the most important reason. In heavy rain and surface water the difference in stopping distance at 70mph between a car with 7mm of tread and one with 1.6 in (from memory) in the order of 10m. And that is before the impact of braking/suspension differences come into play.

One also has to consider the human factor, some people simply press the pedal harder than others, and some will in wet conditions make greater allowance (in terms of not pressing so hard) as others.

2. Make allowances for the following driver to avoid being rear-ended

Indeed and the fact that in low grip the "sandwich" car may be pushed forwards much further than in high grip.

One also needs to consider the need to increase distances to immobile hazards were the doubling is quite unambiguously needed.

Also bear in mind that the audience for the "double your distance" is all drivers and the level of skill varies tremendously. I would suspect that more than 50% of British drivers lack the skill and insight to dynamically adjust their driving (and gap) to meet all the influencing variables - and for them a one size fits all rule of thumb that very conservatively errors on the side of too much is probably the right message.

But to add to your list:

5. The risk of collision is greater in the wet than the dry - it seems logical and prudent that distances should increase in some proportion to the risk of the vehicle in front being involved in a collision.

6. The poor visibility that often accompanies wet conditions increases the likelihood that a lead vehicle will brake sharply. Increasing the gap reduces the extent to which following vehicles will need to brake - helping to reduce congestion cause by excessive braking "shockwaves".

[Astraist]

I am glad to see that my assumptions were indeed to the point. I also consulted with another instructor, and he told me that reduced visibility (amongst other things) can increase reaction times to the braking manuevers of the car ahead. Thank you for the additional points to pounder on.

In heavy rain and surface water the difference in stopping distance at 70mph between a car with 7mm of tread and one with 1.6 in (from memory) in the order of 10m. And that is before the impact of braking/suspension differences come into play.

In fact, I would be surprised if the difference is this small. This strikes me as a conservative estimation with the impact of brakes, suspension, tarmac quality, water depth, run-off, tire quality (irrespective of tread depth) and other factors. I always say that the legal tread limit is under no circumstances sufficient for safety (and I say this as an instructor in a generally dry country). Local experiments did by advanced driving trainers in Israel regarding braking and tires on wet or even on the dry show dramatic differences between good tires and tires that are worn/old/poor quality/underinflated.

One also has to consider the human factor, some people simply press the pedal harder than others, and some will in wet conditions make greater allowance (in terms of not pressing so hard) as others.

My point exactly under the word "driver."

One also needs to consider the need to increase distances to immobile hazards were the doubling is quite unambiguously needed[...]The risk of collision is greater in the wet than the dry - it seems logical and prudent that distances should increase in some proportion to the risk of the vehicle in front being involved in a collision.

My point exactly in my third point above. We agree.

[vonhosen]

http://www.rospa.com/roadsafety/info/ty ... _depth.pdf

[IVORTHE DRIVER]

I was once told to take my normal gap then add the same again for the car in front plus the same again if you are being followed.

By following the above even if the other cars are not leaving a longer gap you can in theory gain at least one and a half stopping distance if it should be needed.

Do you follow that?, seemed simple at the time

[Astraist]

Of course! I train people to maintain not only a proper following distance from the car in front, but maintain a "safety envelope" 360 degrees around the car. If another car tailgates me, I first look at the option of switching lanes to allow him to simply overtake me safely. If not possible, I increase the forward gap by a half (as you suggest) and use signals to encourage him to pass or overtake. If inefficient, I double the forward gap and, if I find a safe place to pull over into I do so to allow him to overtake safely.

[zadocbrown]

I don't know many who consistently leave a 4 second gap in the wet. I wouldn't recommend it myself. Following distance should be dynamic, not fixed.

[MGF]

I thought the following distances, eg '2 seconds' are minimum distances not constant distances one should follow vehicles. And temporarily going under the distance with good reason is acceptable.

[Astraist]

Both right. The gaps should be dynamic. One good example is what Ivor mentioned: increasing the distance to make allowances for the following driver. To tell the truth, the grip levels on a wet surface vary quite widely (depending on the car, speed, wate depth, tarmac quality, run-off) and often three seconds can be perfectly acceptable (there was as thread on this subject around here at the time) but since you cannot anticipate the conditions in the sufficient accuracy, the rule in most advanced driving programs is to use a four seconds gap in the wet.

[ROG]

If following another at the same speed on the same surface then there is no logical reason to increase the gap

For objects going at a slower rate (or stationary) then the distance does need increasing

[Astraist]

In theory, you are correct. But, based on the assumptions of myself and Michael, it's reasonable to increase the gap. The differences between the braking abilities of different cars (including their tire and suspension maintainence, driver skill, response times, etc..) sharpen as the conditions deteriorate. Also, on some surfaces, the rolling resistance is reduced so that the vehicle passes a slightly longer distance from the moment you let go of the gas to the moment the brakes react. Another reason is that reduced visibility can increase reaction times.

Also, in slippery conditions and conditions of lower visibility, there is as greater likelihood of a driver in front being rendered into a stationary obstacle by getting into a collision that halts their progress at once (for which we need additional space and some extra visibility). Alternativelly, the lower visibility is more likely to cause more resistance (more braking and changes of speed/position by the driver ahead) and a bigger gap allows to deal with it better. Even a mild braking effort of the driver in front can cause a "shockwave" which might cause you or the driver behind you to skid somewhat.

It's also the guideline in most advanced/defensive driving programs I know. Two-seconds for dry conditions, four for wet, twelve for frozen...

[IVORTHE DRIVER]

If following another at the same speed on the same surface then there is no logical reason to increase the gap

For objects going at a slower rate (or stationary) then the distance does need increasing

Unless as Mr A pointed out, you need to make allowances for the guy tailgating you!

Stationary objects? If you are too close and the engine drops out of the vehicle in front can you get round it?

I don't think you can ever leave too big a gap.

Of course in Scotland we have more space

[gfoot]

On the flip side, there's rarely much benefit in closing the gap, unless you're planning to overtake.

[ROG]

Stationary objects? If you are too close and the engine drops out of the vehicle in front can you get round it?

To follow that logic we would need to go no more than 5 mph because an oncoming driver might cross into our lane!

[RenesisEvo]

http://www.rospa.com/roadsafety/info/tyre_tread_depth.pdf

Interesting, but numerous variables are not accounted for - e.g. was the same car used each time? Were the brakes allowed to cool between each test? Ambient temperatures? Were they using summer, all season or winter tyres? What effect has different tyre sizes, compounds, different masses of car? Different weight distributions? There's so much to consider, I feel that study is a mere toe-in-the-water exercise.

Also, in my opinion, 5 points on a graph can make a trend, but when it draws the conclusion that a significant reduction in stopping distance occurs below 3mm tread depth, it is only using three data points to draw that conclusion - perhaps not stastistically 'robust' enough, even if the rest of the data supports the trend. Further, they quote percentage changes to 1 decimal place - I'd be impressed if they could prove the results were that accurate - no mention of error margin. In fact, I'd say the report barely meets the standard of a GCSE science report.