Instead of upgrades, healing should be a pick-up that, with a moderate chance, can drop from enemies (for example, regular cars drop health with 20%, trucks drop it with 40%, taxis with 10% etc.). This is suggested simply because you sometimes can upgrade very slowly/get unlucky and not get any healing upgrades, take a lot of damage and eventually die which doesn't seem that much fair.
Speaking of "taxis". Every single car that's not yours and not a garbage truck looks identical with just a color variation. This makes a "Cursed Swarm" upgrade a little vague: What exactly is identified as taxi?