A well known UK TV advertisement shows a car braking from 35 mph and hitting a child pedestrian. The claim is that the car would have stopped in good time if only the driver had been travelling at 30 mph instead of 35 mph. The final claim is that it takes an extra 21 feet to stop from 35 mph when compared with 30 mph. You can download or stream the advert from here.
We've already taken great issue with the implied claim that exceeding the speed limit by 5 mph risks the lives of children. See (click here). And we've proved beyond reasonable doubt that vehicles exceeding the speed limit don't kill pedestrians in 99.92% of incidents. (click here)
This page aims to explain why the obvious fact of greater stopping distances at higher numerical speeds does not appear to affect accident outcomes to any great extent in the real world.
|A further 21 feet?
A 1965 Ford Anglia would take an extra 21 feet to stop from 35 mph (compared with 30 mph). The braking effort assumed is 0.7g and matches the rather old fashioned and pessimistic figures in the Highway Code. Most modern cars will brake at 0.9g (in the dry and on the flat) and will be able to stop in an extra 17.6 feet instead of an extra 21 feet.
But speed isn't the only thing that affects stopping distance. Hills and rain also have big effects. Let's look at rain. In the dry stopping from 30 mph takes around 66 feet. But in the rain stopping from 30 mph takes about 83 feet, that's 17 feet more, and just about the same real increase as the "5 mph over" claim. So perhaps the advertisement should imply that "rain kills"?
Another very important factor in real world
stopping distances is driver attention. Simply observing the hazard and
planning action takes a very significant amount of time. If the driver
wasn't paying attention to the road ahead when the hazard appeared then
the reaction clock doesn't even start until he looks back ahead. We end
up with these four important components:
It's quite obvious that a distracted or slow reacting driver will have a much greater overall stopping distance than one who is simply travelling a few mph faster. You might think that no-one fails to pay attention to the road ahead for 2.5 seconds, but consider this research published by RoSPA (click here) which contains the following alarming statistic on page 5:
So we conclude that poor concentration might add 150 feet to driver response while 5 mph greater speed will add less than 21 feet.
|Safe braking zone
But is stopping distance in feet the overriding factor in deciding the outcome of a potential accident? A more driver oriented view is to examine the distance that a driver must look and plan ahead. You might immediately conclude that the distance that the driver must look ahead is the same as the braking distance, but that's not the most realistic view.
When a driver looks ahead he is always trying to ensure that he can stop within the distance that he knows to be clear. But our driver will not normally be stopping; he'll normally find no hazard or need for emergency braking and he'll continue to drive. To do this he looks ahead beyond the point where he will be in a few seconds. At higher speeds he must look and plan further ahead.
We call the vital component of this distance ahead the "safe braking zone", and we measure it in seconds at the present speed.
At 30 mph the safe braking zone is 1.51
The difference is just 0.12 seconds. Say 1/8th of a second.
If you have any doubts about the calculations, please download our braking physics calculator... (click here)
If we could possibly get drivers to look 1/4 second further ahead we'd get double the improvement that we might get from 5 mph less. This is where the advertisement misses the real point by a mile. Accident risk is far more sensitive to driver observation than it is to speed.
The 1/8th second view is honest and realistic, while the 21 feet view tends strongly to mislead.
|Safety system failures
If we get to the point of panic braking to avoid an accident all the usual safety systems have already failed. The usual safety systems (observation, anticipation and planning) used by all road users (drivers and pedestrians alike for example) largely enable us to avoid one another safely without taking emergency action.
It would be far better if we could reduce the number of safety system failures rather than attempting to reduce the consequences of a failure. Taking the obvious case in the advertisement, we could look at some or all of the following methods to reduce the chances of the incident developing in the first place.
|Is braking the correct response?
In the TV advertisement the driver skids, with brakes locked, straight to the scene of the accident. In the real world a skilled driver could easily have steered to pass behind the child pedestrian.
So for the case in the advertisement, braking was not the correct response.
Other speed limits
If going over 30 mph is so very dangerous, how come we also have 40 mph speed limits?
It does take dramatically further to stop from higher speeds, but to suggest that stopping distance changes with speed are a significant contributor to accidents or accident outcomes is just absurd. You only have to look at the figures for accident outcomes to know that it isn't speed that kills in practise; there are countless millions of cases of potential danger from speed every single day, yet in 2001 only 107 child pedestrians were killed. We would obviously like the figure to be reduced, but asking drivers to slow down by 5 mph is an almost complete irrelevance.
Sometimes drivers use speed unwisely and such cases are to be deplored and discouraged. But the wise use of speed does not depend on an arbitrary speed limit, it depends on all the immediate local circumstances. There's nothing magically safe about 30 mph. There is something magically safe about always being able to stop within the distance that you know to be clear. Emphasizing this vital and somewhat forgotten driver skill could make a real difference.
So we find the advertisement misleading for the following reasons:
Comments on the above are welcome. If there is a demand we will create a comments page. We will be delighted to publish all suitable emails including those whose content we disagree with. Email comment.
Speed limits are a poor substitute for safe speeds
You can't measure safe driving in miles per hour.