I remember the rule of thumb as one car length of space for every ten miles an hour. Now the various departments of motor vehicles have changed their tests/advice to seconds, as in how long it takes one to catch up to where the car in front was...
this is a little hard for me to wrap my brain around, so I looked in up on
www.cartalk.com, and found this quote -
Quoting -
Take this one question quiz:
True or false?
The tailgating rule is to leave one car length for each 10 mph between your car and the car in front of you. (That is, if you're driving at 60 mph, leave 6 car lengths.)
Well, if you said "true", you're wrong. That old rule from Driver's Ed probably doesn't apply in your state. Nearly all states have changed the tailgating law to the two-second rule.
Here's how it works.
Pick a stationary object ahead of you on the roadside. When the car ahead passes that object, start counting off two seconds. (One cappuccino, two cappuccino.) If you got to the object before counting the full two seconds, you're tailgating.
By the way, this is more stringent than the car length rule. For example, at 60 mph, you'd go 176 feet in those two seconds. Using the car length rule, you'd only be about 100 feet away from the car in front of you.
So, do it right. You'll be glad that buffer is there when the transmission falls out of the car in front of you. And, if someone moves into that space--big deal. Slow down and make a little more room.
End of quote.
Well, that means that one length for every 10 miles per hour is too little space to leave..
There were other quotes on other sites about how many feet it takes a car to stop if going x miles per hour.. something like 400 feet if you're going 55 mph - but I didn't write it all down.