You are quite right ! this is a stupid idea. It's only a matter of time before someone hits those height restrictions and causes further damage to the bridge.t would have been better in my opinion to have pipes installed horizontally divided in the middle & swinging from chains. Say a 3" steel pipe suspended every 2' from 1 inch chains so that it would make a very loud bang and likely dent a delivery truck but put no stress the bridge if hit.
Of course a dump truck might just say heck with it - but I fear if a heavy truck impacts that temporary high-limit restrictor it will break free and damage a moto of oncoming car.
I doubt it would seriously damage the bridge but it would not help
4" pipe suspended on 3" chains - as they have on entry points to Road tunnels under the Thames in London - and I know they get hit !!!