Great game! I played it quite a bit and got a high score of 78! I really enjoyed the mechanics of the driving and the game idea of hitting zomies. Quite stress relieving! The sound effects were great as well.
I did notice that there was a issue when pressing Try Again or returning to main menu then back to the game. I believe this has to do with the Time.timeScale being set to 0 but not being set back to 1 when trying to play. I was able to bypass it by double pressing [esc] to pause and unpause the game.
A game feature that I think would be nice is to increase the time by a set amount based on zombies you kill, so if you kill one zombie you get 0.01 seconds more or something! Would allow for longer gameplay than 20 seconds.
Great job though!