why not just have cars with rear blast shields and we'll have gimballed grenade launchers on the road side.

That requires too much infrastructure. Just have the cars carry their own grenades. If you can make them tiny nuclear bombs (Californium-251 might enable this) then you could achieve amazing range.

would be a solution to tailgaters?

Unmanned blimps and lasers could do it.

In a far out, sci-fi kind of world, huge solar powered blimps hover in fixed traffic patterns, collecting more solar power than they use, and relay fractional amounts of collected surplus energy down to parasite vehicles, that assume a formation for refueling laser targeting, within the blimps flight pattern.

The cars accept the laser energy in a beam collector, for energy conversion, netting a bonus as recharge, and then resume independent travel as their intended destination requires.

Predictive consumption graphs can estimate optimal routes, for different charge requirements before leaving, optimizing for fastest time, best blimp paths, least energy consumption, traffic congestion avoidance or whatever.

beam collector sounds safer than mini furnace :)

Microwaves. Remember to put the wire meshes to your windows before entering the powered section...