The thinking distance all depends on the reaction
time of the driver to whatever obstacle is causing them to start braking. People's reaction
times are all different so it is hard to give a general thinking distance even at a certain
speed.
Lets look at some possible reaction times:
Top athletes at a sprint
start have reaction times between 0.110 and 0.170 seconds. In fact the minimum reaction
time before they are deemed to have made a false start is 0.100 seconds. If they move within
this time after the gun is fired it is a false start.
Baseball batters have just under 0.2 s
to react to the ball being thrown before they have to hit it.
The best reactions for
ordinary people in everyday life to simple stimuli are:
0.19 s to a light and 0.16 s to a
sound.
However these times all depend on a number of
factors:
Alertness
Expectation that something is about to happen (like the athlete at
the start)
Tiredness
Age
Distractions
Blood alcohol content
Now to a
car driver. Normally even an alert car driver is not generally expecting a need for sudden
braking and so the reaction times are increased to around 0.7 s. This may be even longer if
the driver is adversely affected by the some of the factors in the list.
So lets assume
the reaction time of 0.7 s.
Distance = speed x time and so at 15 m/s Thinking
distance = 15x0.7 = 10.5 m
The Highway Code gives 9m at 30 mph (13.6 m/s) and
12 m at 40 mph (18 m/s) and these results seem to fit in with my answer for 15
m/s.
For someone on a motorway travelling at the maximum legal speed of 70 mph
(31.8 m/s) the thinking distance would be 22.3 m. This is before any braking could
start!