Sloping roads before checkpoints
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Before checkpoints, there's usually a traffic jam. People stop their engines, then after a few minutes, they start them, drive a few feet, and turn them off again. Or they keep the car running all the time.
My idea is to build in a slight gradient before checkpoints and other places that are known to cause traffic congestions. People can turn off their engines, and if the traffic moves on a tiny bit, they can simply loosen their handbrakes without starting the motor.
Of course it will cost a lot of energy to build the gradient, but in the long run, it should definitely pay off. Also, when building new checkpoints, the natural fall of the landscape might be considered.