Road departures are one of the most deadly crash modes, accounting for nearly one third of all crash fatalities in the US. Lane departure warning (LDW) systems can warn the driver of the departure and lane departure prevention (LDP) systems can steer the vehicle back into the lane. One purpose of these systems is to reduce the number of road departure crashes. This paper presents a method to predict the maximum effectiveness of these systems. Thirty-nine (39) real world crashes from the National Automotive Sampling System (NASS) Crashworthiness Data System (CDS) database were reconstructed using pre-crash velocities downloaded for each case from the vehicle event data recorder (EDR). The pre-crash velocities were mapped onto the vehicle crash trajectory. The simulations assumed a warning was delivered when the lead tire crossed the lane line. Each case was simulated twice with driver reaction times of 0.38s and 1.36s after which time the driver began steering back toward the road. In addition, each case was simulated a third time, assuming it was equipped with LDP, which removed the reaction delay of the driver. The LDW or LDP system was assumed to be effective in preventing the crash if the point of impact of the original crash was not within the predicted return trajectory. Our preliminary findings were that a maximum LDW effectiveness would range between 16.7% and 21.5%. Maximum LDP effectiveness was 24.3%. This is the first method of its kind to investigate the effectiveness of LDW and LDP using EDR data.