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This paper provides a detailed investigation into two potential forms of onboard road departure warning systems. A method known as time-to-lane-crossing (TLC) is compared with rumble strips placed a fixed distance from the road edge, and is found to provide enhanced performance over such static rumble strips in terms of reduced false warnings and increased warning anticipation. A new approach, called the variable rumble strip (VRBS), is proposed, which is an onboard electronic implementation of the static rumble strip where the warning threshold is allowed to vary according to the risk of the vehicle departing the road. Performance of the fuzzy-logic-based VRBS system is similar to that of the TLC-based approach, but requires less sensor information, making it more feasible in a vehicle application. Performance is measured in terms of hits, misses, and false alarms based on a validation warning set comprised of either static rumble strip generated warnings or by a warning set generated by subjective interpretation of the road departure criticality. The algorithms are tested on 2-h driving simulator runs by 12 drivers. This paper also includes an extension to the VRBS system involving the use of an estimate of driver lane-keeping performance to alter the VRBS threshold adjustment.