Timer. I will give it a thought.
The concept behind this setup is: main heroine runs away on tops of the cars, from officials in pursuit. The cars will vary in size and type and since run will be from left to right, vehicle flow will be from right to left. Any jump made between cars and on collision with asphalt will be lethal (or not, depends on gameplay). I'm planning to have something like 18 different cars, either placed in fixed queue or placed randomly. Player needs to take a long road from one side of the layout to the other and the flow must be dense enough to provide platforms to jump on; wrapping was the way to save some cpu.