I think its useless and to hard to implent, I like you idea, but its not going to work.
In the Netherland we have many and I mean many "drempels", that are little bumbs in the road that prevent people to drive to hard. Also it is very depending on how accurate the device is, some device have good accuration and some other thinks that you are falling 10KM without moving you hand.
Its also to hard to implent it, dev must do resource how the device shakes and what typ of road it is. What if you dont have a holder and you going hard right or you let it fall ? Phone flies away and waze makes a hazard or bad road condition.
Also if you cant drive that good or you have mechanical problems that the car shakes when you give gas? I think its just to complex to intergrate it into waze.
New car's have a system tat when you make a emergencybrake, the alarm light are going on, maybe waze can inplent this with making a 'car stopped', but also then you must avoid the phone is fly in your car or you let them drop.
Sorry for bad english