Engineers tried to make a "forgiving roadside". They tried to make suburban+city streets like interstates: straight, wide, and with as few trees and poles and people as possible. But that only encourages cars to treat city streets like interstates.
"Those curves, trees, and benches that engineers removed had actually been making drivers slow down to avoid the risk these hazards presented—without all that, drivers felt less at risk and more in control" so they drove faster & killed more people.