Transportation and Vehicle Safety: Calculating speed differential, target speed, distance over time

Question
Hi Terry, I am trying to determine how best to estimate the speed of another vehicle passing me. Assuming I have a constant rate of speed, and can estimate the separation of distance over time, can I just use the conversion of FPS to MPH of .6818? Example: I am driving at 70MPH. Another vehicle overtakes me and passes me. Within 3 seconds the vehicle is 60 feet ahead of me. Taking 20FPS multiplied by the conversion of .6818 the other vehicle is traveling at approximately 83 MPH, correct?

Answer
Hello Scott and welcome to AllExperts!

CONVERTING MPH INTO FPS:
You multiply the MPH by 1.467 which will give you FPS.

CONVERTING FPS INTO MPH:
You multiply the FPS by 0.6818 which will give you MPH.

While the math is correct, your estimate regarding speed must be constant in order for your assumption to work. If the vehicle is decelerating or accelerating at all,  your estimate will be off.

In your example. you estimate that the vehicle is traveling 13.64 MPH faster than you (70 MPH), which is approximately 83.65 MPH.

Judging distance and accurate elapse of time must be spot on, but assuming all your observations are exact, then your have properly arrived at the vehicles target speed. Good job!

Have a great day!

Terry