how about words only, big block letters, on the back.
1.ZERO emission (i mean the bike, not the rider)
2.need water, not gas NEED COCA COLA, NOT GAS
3.BURN FAT, NOT GAS
4.THANK U, FOR POLLUTING THE AIR
5.MY OTHER BIKE IS A BIKE
6.THANK U, 4 RESPECTING OTHER ROADUSERS
7.BEATS WALKING
8.BETTER THAN WALKING
9.WHICH ONE IS FAST, YOU OR YOUR VEHICLE?