An on-site roadway warning system for alerting a driver of an approaching vehicle of a need to brake the vehicle. A transceiver radiates electromagnetic energy toward the approaching vehicle and receives reflected energy from the vehicle. A processor processes data about the reflected energy in relation to data about the radiated energy to yield data disclosing both range of the approaching vehicle from the transceiver and rate at which the vehicle is approaching the transceiver. The processor processes the data disclosing range of the approaching vehicle from the transceiver and rate at which the vehicle is approaching the transceiver in relation to the stored data, and activates a signaling device to signal the approaching vehicle when the data disclosing range of the approaching vehicle from the transceiver and rate at which the vehicle is approaching the transceiver correlates with stored data that identifies a potentially dangerous combination of range and rate.