"Why do civil and traffic engineers sign off on deadly, community killing street designs?"

Insular cliquishness contributes to the problem, doesn't it?

And I'm not referring to the engineers, just to their political paymasters.

Yes, the engineers are the problem. They need to be the solution, by "this is only a test" at Daily Kos

 ... Professional engineers (PEs) are supposed to be our experts. We require their sign off to build things. If politicians or businesses or officials asked for deadly airplanes, would aviation engineers comply? Of course not. Aerospace engineers investigate air crashes and improve aircraft safety. Their goal is zero deaths and injuries. So why do civil and traffic engineers sign off on deadly, community killing street designs? ...

 ... Instead of safety, the real traffic and civil engineering goals are speed and throughput: getting more cars flushed through your place from elsewhere to elsewhere. People who live or work or play along the streets are just damned inconveniences. In Maryland legend (and probably in fact, although I don’t have proof), Montgomery County traffic engineers actually made up ballcaps with the initials FSS, which they wore to meetings. It was an inside joke, I suppose. “Fuck [the] Side Streets.”

Anyone who has been to a community planning meeting knows that engineers can be very intimidating. It's even worse when they don't even consider then needs of the community over their perceived need for ever-faster traffic.