A few bugs:
if you lose and restart and the enemy cars don't move or respawn (you have to esc and restart to fix)
if you shoot in the grass area your bullets don't replenish.
if you bump an enemy car it will careen off the stage never to be seen again and none will spawn after
But, the concept is straight forward. I like the fact you have a pause screen, restart button, health and enemy counters. Add some sound effects and a few bug fixes and tweaks, throw some random barriers and places the enemy cars could spawn and you'd have something pretty solid