Stopping

I read a magazine article this week where a journalist attacked the stopping distances quoted in the Highway Code.  Modern braking systems and tyre compounds, he claimed, have shortened the distances in which we can bring our vehicles to a halt.  The whole thing, he seemed to be suggesting, was so out of date it was next to useless.

Yes, I thought, braking distances for some vehicles may have shortened, but the overall picture is not quite that simple.  The Highway Code may be old in detail but the general message still holds true.  Here`s why.

The Highway Code uses a short reaction time (0.7 seconds). 

The distance a vehicle will travel while the driver reacts lengthens as reaction time increases.  Most drivers take a little longer, in fact on a motorway reaction times can reach 1.5 to 2 seconds. 1 second is considered a reasonable representation, but by no means average.  Coefficients of friction between the tyres and the road have increased with modern compounds and construction.  The Highway Code uses 0.65, which is no longer typical for most cars - they can now exceed 0.8. Many trucks and buses, however, still have tyres that only achieve about 0.65. 

A speed of 30mph is used for the calculations above. The deceleration rate and reaction time calculated is the same for all speeds quoted in the Highway Code.  The figures for s, reaction distance of 9 metres and braking distance of 14 metres, are taken from the code.



The Highway Code stopping distances compared with those representing greater tyre friction coefficients and increased reaction times.  

The end result is similar stopping distances.  It should be noted that impacts that occur at the same point along these distances will vary in speed between the two sets of graphs (e.g. at 50mph, the Highway Code figures show the vehicle braking before 20m has been covered. With a longer reaction time, the driver doesn`t start braking until after 20m).  The dotted line on the Highway Code graph illustrates the squared relationship between energy (work) and speed. The braking distance (red) at 60mph is about 4 times that of 30mph, despite the speed being only doubled.