  We present an economics-based method for deciding the optimal rates at which
vehicles are allowed to enter a highway. The method exploits the naturally
occuring fluctuations of traffic flow and is flexible enough to adapt in real
time to the transient flow characteristics of road traffic. Simulations based
on realistic parameter values show that this strategy is feasible for naturally
occuring traffic, and that even far from optimality, injection policies can
improve traffic flow. Our results also allow a better understanding of the high
flows observed in ``synchronized'' congested traffic close to ramps.
