A motion activated trigger circuit would be child's play for a mechanical engineering student, and would be effective against a mobile target, once attached and armed.
Depending on the type of trigger, anything from starting the bus to going up a steep hill or hitting a bump would cause the bomb to go off.
I've seen the small reed switches that are supposed to detect when you close a notebook screen be defective present a permanently closed circuit. Putting one of those in your bomb would be very bad.
The Palestinians seem to blow themselves up by accident on a fairly regular basis and they have a lot of experience with these devices.