Walkers are far worse than cars. They stop, turn, and reverse instantly.
Helmets are good. They can help prevent you from cracking your skull when you slip on wet leaves or grass, ice, streetcar tracks, and flexis.
They are not designed to mitigate a head hitting the windshield of a high speed car. (their design speed is something like 10mph- ie the speed your head will be at when you fall). They might however be useful if that car sideswipes you, and you hit your head on the ground.