Alarms are useful if you have people to react and in a position to react.
Can the 'defenders' intercept a fast moving object in time to halt the attack. Is the attack likely to take place at a time when a patrolling security vehicle is near enough to react and intercept?
Might a [police or civilian] police air observer be able or equipped to halt a an armed intruder let alone a vehicle of any kind travelling at 30-50mph when alerted? Ask Surrey.
As was proven, sensors might be useful as part of a collective of physical obstructions but in the end you still need the obstructed path/wall etc and some sort of ultimate threat. The latter of course is not going to happen in the UK.