[Physics] Why does this radar speed sign show a decreasing speed, despite the fact that I’m accelerating?


Everyday when I'm going back home from college, I pass by this big radar speed sign and I've noticed that it does a peculiar thing. The sign is placed on a downward slope of the street. So say I'm driving on 35 MPH, and I let my foot off the gas pedal completely, the street makes my car go a little faster since it's steep, and my speedometer reads 40 MPH, but the speed on the radar goes down instead of up, it gradually goes down and shows 31 or 32 MPH.

Can anyone explain this to me? Or is the radar just faulty?
And if the radar is not faulty then which speed am I actually going?

Best Answer

It's most likely that the radar gun is just locking on to a different vehicle. Depending on how it's filtering and processing the received signal, it could even be measuring the speed of a car going in the opposite direction.