Bollards and concrete planters are excellent for stopping a lot of mass going very fast, this is true. One thing to keep in mind about the threat of "flying ca

Bollards and concrete planters are excellent for stopping a lot of mass going very fast, this is true. One thing to keep in mind about the threat of "flying car attacks" is that weight requirements of staying in the air generally remove the "mass" part of the equation. Someone could pack a flying vehicle full of explosives, but explosives that pack a big punch and that are compact & light weight are pretty regulated. Explosives that are easy to acquire tend to have more weight (fuel oil & fertilizer, for example). You could only pack so much in a vehicle before it couldn't take off anymore. In general, I don't think flying cars would be any more of a risk than light aircraft, which have been around for decades without very many incidents of successful terrorism. As an example, about a decade ago, and anti-tax nut flew a light aircraft into an office building that housed local IRS offices in Austin. It caused a lot of damage, but not serious structural damage and there were no fatalities other than the pilot.