How do you use a fixed roadside marker to check the time gap behind the vehicle ahead?

Correct answer: Count from when its rear passes the marker until your front reaches it

Why

Choose a fixed point ahead. Start counting seconds when the rear of the vehicle ahead passes it and finish when the front of your vehicle reaches the same point. PEI’s handbook recommends at least three seconds in normal conditions, with more time when conditions or the vehicle being followed require it. If your gap is too short, ease off and check again. This guideline does not replace the legal requirement for a reasonable and prudent distance.

Verified against Prince Edward Island Driver's Handbook · Speed, Space and Driving Skills · p. 98, 99, 101