The amount of Doppler effect depends on speed - the faster vehicle will show more Doppler shift.
The Doppler effect causes the received frequency of a source (how it is perceived when it gets to its destination) to differ from the sent frequency if there is motion that is increasing or decreasing the distance between the source and the receiver.
This effect is readily observable as variation in the pitch of sound between a moving source and a stationary observer. The sound a race car makes as it rushes by, whining high pitched and then suddenly lower. Vrrrm-VROOM. The high pitched whine is caused by the sound waves being compacted as the car approaches you, the lower pitched VROOM comes after it passes you and is speeding away. The waves are spread out.
You also can observe the Doppler effect when you are moving past a sound source that is standing still.
Suppose you were riding in a school bus and passed a building with a ringing bell. The pitch would sound higher as you approached the building and lower as you rode away from it. The Doppler effect happens any time the source of a sound is changing position compared with the observer.
It occurs no matter whether it is the sound source or the observer that is moving. The faster the change in position, the greater the change in frequency and pitch.
To learn more, refer to this study leaflet about sound (pdf).