For the less shifty crowd:
Lets say your decal is emitting red at 650 nm and the speeder observes it as blue at 450 nm.
ëemitted –
ëobserved)/(
ëemitted) =~ v/c where ë = lambda
v =~ 0.308 c ignoring special relativestic effects
v =~ (0.308) * (3 x 10^8 m/sec) = 9.2 x 10^7 m/sec
v =~ 210,000,000 miles per hour closing speed.
Doppler blueshift officer, the light wasn’t red.
Best,
Grady