Wouldn't it be cool if you could move from point a to point b without atracting half the population of zombies to your car? Well with bikes you could! It would be silent and fun to use, speed could be affected by your weight, you could have the option to make the bike heavier or lighter, you could have lights and a bell, but also it would tire you out! You can't just pedal to eternity, you would need to take breaks and such, maybe drink some water. You could have part of the usefullnes of cars without so much the noise!
Of course you would have limited storage space (Only a front casket) but it would be something. Also you wouldn't need gas but you would need to oil it and maybe put back the chain when it goes out by just riding it. It could also have a little thing in the backwheel for another player to ride with you, maybe also a little water bottle thingy in the body of the bike if they are sporty bikes. And also talking about sport bikes you could have different bikes! City riding bikes with a basket in the front, sport bikes wich are fast and have a watter bottle thingy, BMX that go really well in concrete and also mountain bikes for the forests and such!
All in all i think bikes would be a perfect thing to add to the game and it would also let you have more fun with the mechanics skill so yeah, i would love to see it in the game!
