Cars would be overpowered IMHO. Too fast, likely too much carrying capacity, and makes the player relatively safe. If added, there would need to be serious consequences, like attracting every zed for blocks around. Make it so bad, players have to park outside of town and leg it into town, or face a near overwhelming horde. I'm a fan of bicycles. Slower, limited carrying capacity, and low noise. Also, no gas, so you just need to keep a supply of food on you. Too keep it from being too overpowered there would need to be some negatives, like getting hungry or thirsty faster.