THE EFFECT OF DAYTIME USE OF MOTORCYCLE HEADLIGHTS AND TAILLIGHTS ON MOTORCYCLE ACCIDENTS, NOTICEABILITY, AND ELECTRICAL SYSTEMS WAS EVALUATED. THERE WAS A SIGNIFICANT DECREASE IN DAYTIME MOTORCYCLE ACCIDENTS IN FOUR STATES WITH DAYTIME MOTORCYCLE HEADLIGHT LAWS, GIVING AN ANNUAL PROJECTED REDUCTION OF OVER 7,000 MOTORCYCLE ACCIDENT INVOLVEMENTS. ANNUAL ECONOMIC SAVINGS WOULD RANGE FROM $7.50 TO $10.90 PER REGISTERED MOTORCYCLE. DAYTIME OPERATION OF MOTORCYCLE HEADLIGHTS SIGNIFICANTLY INCREASES NOTICEABILITY, AND DRIVERS CAN POSITIVELY IDENTIFY MOTORCYCLES SOONER AND AT GREATER DISTANCES. STANDARD TAILLIGHTS, EVEN MODIFIED BY INTENSITY, HEIGHT, NUMBER, OR CONTRASTING BACKGROUND, DO NOT INCREASE NOTICEABILITY. MOST MOTORCYCLES APPEAR TO HAVE ADEQUATE ELECTRICAL SYSTEMS FOR EXTENDED DAYTIME HEADLIGHT AND TAILLIGHT OPERATION. INCREASED ANNUAL MAINTENANCE AND REPAIR COSTS FOR MOST MOTORCYCLES WILL BE NEGLIGIBLE WITH BENEFIT-TO-COST RATIOS GREATER THAN 1. /HSL/