EXPLORATION OF VOLUME-CAPACITY-SPEED RELATIONSHIPS ON URBAN ARTERIAL STREETS

AN INVESTIGATION WAS CONDUCTED TO RELATE TRAVEL SPEEDS TO VARYING CAPACITIES AND VOLUMES FOR SUBSTANTIAL LINKS OF URBAN ARTERIAL STREETS. DATA WERE COLLECTED ON 6 URBAN ARTERIAL STREETS IN RALEIGH, NORTH CAROLINA. POTENTIAL BOTTLENECKS ALONG EACH ROUTE WERE EVALUATED TO DETERMINE THEIR EFFECT UPON TRAFFIC FLOW. EACH SITE WAS THEN ANALYZED TO ASCERTAIN OVERALL VOLUME-CAPACITY-TRAVEL SPEED RELATIONSHIPS WHICH WOULD REFLECT THE DRIVERS' REACTION TO OPERATIONAL AND ENVIRONMENTAL CONDITIONS ENCOUNTERED. INDIVIDUAL SPEED-VOLUME/CAPACITY RATIO CURVES ARE PRESENTED FOR EACH TEST SEGMENT. FOR THE 6 SITES STUDIED NO VOLUMES REACHED SUFFICIENT LEVELS TO CAUSE A BREAKDOWN ON THE FACILITY. A SIGNIFICANT REDUCTION IN AVERAGE OVERALL TRAVEL SPEED IS APPARENT ON THE 45-MPH FACILITIES AS THE VOLUME/ CAPACITY RATIO INCREASES IN LEVEL OF SERVICE. A LESS SIGNIFICANT REDUCTION IN AVERAGE OVERALL TRAVEL SPEED IS APPARENT ON THE 35-MPH FACILITIES AS THE VOLUME/CAPACITY RATIO INCREASED IN LEVEL OF SERVICE. NO MAJOR VARIATIONS OR REDUCTIONS IN AVERAGE OVERALL TRAVEL SPEED ARE EVIDENT FOR THE MID-RANGE OF VOLUME-CAPACITY RATIOS. FOR V/C RATIOS IN THE GENERAL RANGE OF 0.4 TO 0.6, THERE APPEARS TO BE SOME SMALL INCREASE IN TRAVEL SPEED COMPARED TO THE SPEEDS AT RATIOS 0.2 TO 0.4. AT V/C RATIOS BETWEEN 0.1 AND 0.6 THE MOST SIGNIFICANT VARIABLE AFFECTING OVERALL TRAVEL SPEED ON URBAN AND SUBURBAN ARTERIALS APPEARS TO BE THE SIGNAL PROGRESSION AND TIMING. IT IS CONCLUDED THAT IT IS NOT FEASIBLE TO SHOW ANY TYPICAL SPEED VOLUME CURVE FOR URBAN ARTERIALS REPRESENTING ACTUAL VOLUMES OBTAINED UNDER IDEAL CONDITIONS.