Everyone takes a finite time to respond to any stimulus, whether it's the brake lamp from
the vehicle in front at 110km/h on the freeway, touching a hot saucepan on the
stove or whatever.
There's the short time for the nerve impulses from your senses
to travel to your brain, the time for your brain to respond and then a further
short time for outgoing nerve impulses to travel to your limbs and stimulate the
muscles to produce your reaction.
These three delays or 'latency times' are usually lumped
together into a single quantity known as your reaction time: the total time
taken for you to actually respond to such a stimulus.
Your reaction time varies depending on whether you respond with
your hand or your foot. It also depends on your state of health, alertness,
psychological outlook and whether you have recently taken drugs or alcohol.
The reaction time for a normal healthy adult seems to vary
between about 150 and 300ms (milliseconds) for a hand response and between 400
and 800ms for a foot response (eg, hitting the brakes). If you are driving a
vehicle and your measured reaction times are significantly longer than these
times, you are an "accident waiting to happen".
You don't need to be a rocket scientist to work out why.
Consider driving at 70km/h. At that speed, you're travelling a distance of 19.4
metres every second or almost two metres in each 100ms. So if it takes you (say)
500ms to respond to an emergency by stepping on the brake pedal, your car will
travel almost ten metres before the brakes can even begin to slow you down.
Some safety experts have been lobbying for years to make
reaction time testing mandatory for driver's licence renewals. It hasn't
happened yet - but in the meantime you can measure the reaction time of all your
driving friends, to judge whether they should be on the road or
not...
