What's the 3 -Second Rule?
Driving instructors teach new drivers to use the "3 second rule." When driving, pick a non moving thing along the road (like a speed limit sign, or something painted on the road) and when the vehicle in front of you passes that thing,start counting in your head. Count slowly, saying "one-alligator, two-alligator, three-alligator" and note when your vehicle passes that object.
You want to be able to count to 3 in the time it takes your car to reach the object, as that would mean you have enough time to stop your vehicle if the one in front of you came to a sudden stop. This is a flexible standard, and this makes it helpful in many situations, like when travelling very fast on the highway, or going more slowly down city streets.
When you are stopped at a stoplight, you don't need to leave a full car length between you and the vehicle in front of you, but a good "rule of thumb" is to leave enough space that you can see the back wheels of the car in front of you on the pavement.