Which is the mathematically safest distance between two vehicles at 55 m.p.h. on an interstate Hiway?

Question

i.e. The 3 sec. rule? 3 secs between cars. Or the car length rule? i.e. 1 car length for every 10 m.p.h.

Answer 1

3 car legths is the right answer. What ever you feel safest at.

Answer 2

I tell my students 1 car length for every 10 MPH....No one seems to argue with that..

Answer 3

Driver courses now do not refer to car lengths - all cars are different and who can judge that at highway speeds? Instead, they say pick a spot that the other vehicle crosses and then count how many seconds until you reach that spot. It should be at least three seconds, long enough for you to react.  But going along with this, how good are your brakes? Do you have ABS? What if you don't and the car in front does? It will stop faster so you should allow a little more time.

If you leave 5 1/2 car lengths open, some idiot will cut in front and soon you will have only one or two. Oddly enough, it then is safer to follow a little closer to preclude being cut off.

Answer 4

5 and 1/2 cars behind.

Answer 5

4 seconds at a minimum.  One has to allow at least one to two seconds for the "Is this really happening?" decision making process to occur.  Then, at least 2 to 3 seconds to do something, i.e. hit the brakes, steer car to avoid, or both.  Sadly many drivers I encounter each day on I-65 believe 1 car length away from my rear bumper is not close enough, even at 75 miles an hour.  Here in the US, we do not DEMAND driving skills from people when they get  their driving licenses.  I see terrible decision making EVERY day I drive on the roads.  So few drivers have the thought of "What if.............?"  My Dad taught me to look at a situation and think "What if.....?"  Also he taught to ALWAYS have an out when I drive.  Do not allow yourself to get boxed in without an escape route.

Be skillful and safe out there.

Answer 6

Its the one car length every 10 mph as a general rule, but that changes do to light and weather conditions, also traffic.

Answer 7

Either rule works.    

At faster speeds,  more distance between vehicles  is needed to provide the allotted time.