INCIDENT on bridge. Be Aware. 30 seconds later - LOCKDOWN - Terrorist attack. Draws weapon. Prepares for further eventuality.
This type of attack by its very nature is nearly impossible to stop casualties, there's no warning, no counter-intelligence that really can pick up on dodgy things etc and it happens very quickly.
The fact that he "only" got over the bridge, crashed and just inside the grounds shows how quick the response was and it should be applauded.
I find your implied suggestion there was a failing of the police response frankly abhorrent.