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A
Always check mirrors every two seconds
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B
A following-distance guideline — choose a fixed point, when the vehicle ahead passes it count two seconds; if you reach that point before counting to two, you are following too closely
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C
Acceleration from a stop should take at least two seconds
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D
Two seconds is the maximum time allowed at a stop sign
Why this is the answer
The TWO-SECOND RULE is a simple method for maintaining a safe following distance that automatically adjusts for speed — because at higher speeds, two seconds represents more physical distance. HOW TO APPLY IT: Pick a fixed, stationary reference point (road sign, shadow, overpass, crack in the pavement); watch the vehicle ahead pass that point; count 'one-thousand-one, one-thousand-two' (approximately 2 seconds); if your front bumper reaches that reference point before you finish counting, you are following too closely. WHY IT WORKS AT ANY SPEED: At 30 mph you cover approximately 88 feet in 2 seconds; at 60 mph you cover approximately 176 feet. The faster you go, the more physical space the rule automatically builds in — matching the reality that faster speeds require more stopping distance. LIMITATIONS: Two seconds is a MINIMUM for ideal dry conditions; in rain, fog, heavy traffic, at night, or with a large/heavy vehicle, increase to 3-4+ seconds; for drivers under 21 or inexperienced drivers, 3-4 seconds is recommended; tailgaters are one of the leading causes of rear-end crashes. WHAT THE RULE DOESN'T COVER: It measures reaction-and-braking distance but doesn't account for perception time (seeing the hazard before it's in your reaction window); total perception-reaction-braking time means 3-4 seconds is actually safer as a standard practice for new drivers.
Source: State DMV handbooks, Speed Management, Following Distance