Question

At 10:17 a.m., you pass a police car at 55mph that is stopped on the freeway. You pass a second police car at 55mph at 10:53 a.m., which is located 39 miles from the first police car. If the speed limit is 70mph, can the police cite you for speeding?

Answer 1

Regardless of the speed at which the two police cars were passed, the really important thing is the average speed between the two points. The distance traveled between these two points (x) is 39miles and the time between them is:

$\Delta t = 10:53-10:17$

$\Delta t = 36min$

$\Delta t = 0.6hours$

The speed limit is 70mph

Average speed is the change at the distance in this time, then

$\bar{v} = \frac{x}{\Delta t}$

$\bar{v} = \frac{39}{0.6}$

$\bar{v} = 65m/s$

That means the average speed is less than the speed limit. The police cannot do anything against the average speed that was done during the two points.