Hi helshea
I think your fear of disasters beneath motorway bridges is quite a popular fear and augmented by a screwed half vision in case a brick comes hurtling through your windscreen. And could be that the half-closed eyes and the wince are the major factors in resultant collisions near bridges. Apart from banning juveniles from stopping on bridges overlooking traffic, and using TV to entrap offenders there is little that can be done except hold the parents of offenders responsible.
As for jumping I remember being called out as duty officer for the borough where a lady, well-known to my department, was threatening to jump off a a bridge crossing a busy main road. Two young policemen were there not knowing quite what to do. I went through and spoke to poor Marjorie who was reading from her bible. I managed to persuade her to come with me to Saxondale Hospital where she was admitted for observation under section 29 of the Mental Health Act 1959.
On paper it seems a callous response to the poor lady's difficulties, but clearly she was a danger to herself and to others. The coherent part of her day was that she wanted to jump onto a clear part of the road - but the traffic was so heavy that there was no clear part, and it was making her cross.
But yes, dear helshea, it bothers me sometimes. Thankfully I see very few people waiting to jump. Best thing is not to look.