The Doppler effect is what makes red cars go faster than blue ones ... this is known as "red shift".
static object
A static object transmitting waves of constant frequency. The wave-fronts are shown at constant time intervals. All the wave-fronts are centred on the object since it isn't moving.
slow-moving object
If the object moves (slower than the wave speed) then the wave-fronts ahead of the object emerge closer together than the static case and the wave-fronts behind are farther apart. This is the doppler shift; if the object is approaching then the wavelength it transmits is reduced, if the object is receeding then the wavelength it transmits is increased.
If the object is a red car then, because red cars have a longer wavelength than blue cars, it can travel much faster than a blue car before its transmitted wavelength is reduced to a dangerous level - energy being inversely proportional to wavelength.
fast-moving object
If the object moves (faster than the wave speed) then all the wave-fronts are left behind the object. These wave-fronts interfere with each other and this interference is responsible for the effect associated with super-sonic aircraft known as "sonic boom".
It is theoretically possible for red cars to produce a sonic boom but this has never been demonstrated in practice because there are always too many other (non-red) cars about for them to get a clear run.