Because a person likely to set off the alarm is likely to be driving erratically, meaning a high probability of being DUI. Not everyone who sets off the alarms

Because a person likely to set off the alarm is likely to be driving erratically, meaning a high probability of being DUI. Not everyone who sets off the alarms will crash, but there could be a high proportion of them who could easily harm someone else on the road. Although by the looks of this system it is to be similar to a rail system, so I'm unsure if a side-on-side collision would even be capable of derailing this and causing serious harm. Honestly, I'd be more worried about car-on-car crashes under these. I've seen how stupid people are driving under bridges. Despite no shape-change in the lane, people tend to drift away from any walls or support pillars as though they're afraid of hitting them, and I've seen it to the point of them cutting other drivers off.