DEVELOPMENT AND TESTING OF A MULTIPATH-ASSIGNMENT TECHNIQUE

IN MOST CONVENTIONAL TRAFFIC-ASSIGNMENT TECHNIQUES, THE TRAVEL BETWEEN TWO POINTS IS ASSIGNED TO THE MINIMUM PATH BETWEEN THEM. EVEN IN CAPACITY-RESTRAINT ASSIGNMENT, IN WHICH A CAPACITY FUNCTION OR SPEED-FLOW RELATION IS USED IN SUCCESSIVE ASSIGNMENT RUNS TO MODIFY LINK SPEEDS FOR USE IN THE ASSIGNMENT, MINIMUM-PATH ASSIGNMENT IS USED IN EACH RUN. A DISTRIBUTION OF TRIPS OVER SEVERAL PATHS IS USUALLY OBTAINED BY COMBINING THE RESULTS OF SUCCESSIVE ASSIGNMENT RUNS. A MULTIPATH-ASSIGNMENT TECHNIQUE HAS SEVERAL ADVANTAGES OVER CONVENTIONAL ONES. IN NETWORKS WITHOUT RESTRAINT, EMPIRICAL STUDIES HAVE SHOWN THAT TRAVEL BETWEEN TWO POINTS USUALLY DISTRIBUTES ITSELF OVER SERVERAL ROUTES. FURTHER, MINOR NETWORK CHANGES CAN OFTEN PRODUCE MAJOR CHANGES IN ASSIGNED VOLUMES WHEN MINIMUM-PATH ASSIGNMENT IS USED. IN CAPACITY- RESTRAINED ASSIGNMENTS THE MULTIPATH TECHNIQUE ENABLES CONSIDERATION OF EACH ASSIGNMENT INDIVIDUALLY, SO THAT (1) ALL THE VOLUME ON A GIVEN LINK IS ASSIGNED AT THE SAME SPEED, AND THE CONSEQUENT LINK VOLUMES ARE CONSISTENT WITH THE SPEED--FLOW RELATIONSHIP FOR THE LINKS, AND (2) EXTREME SPEED OSCILLATIONS AND UNREALISTIC PATHS PRODUCED BY THEM ARE LARGELY EXCLUDED. IN THE METHOD DEVELOPED AND TESTED THE ALGORITHMS USED FOR BUILDING THE MULTIPLE-ROUTE TREES ARE THE SAME AS THOSE USED IN MOST CONVENTIONAL MINIMUM-PATH ASSIGNMENT PROGRAMS, EXCEPT THAT A DIFFERENT SET OF LINK COSTS IS USED TO BUILD EACH TREE. WHEN A LARGE NUMBER OF TREES IS BUILT, THE PATHS WILL BE DIVIDED AMONG THE FEASIBLE ROUTES, WITH THE LARGEST NUMBER NORMALLY ON THE MINIMUM PATH AND THE NUMBERS ON ALTERNATIVE PATHS DECREASING AS THE EXTRA COST INVOLVED IN USING THEM INCREASES. TESTING AND EVALUATION OF THE PROGRAM WERE CARRIED OUT IN TWO STAGES. IN THE FIRST, UNRESTRAINED ASSIGNMENTS WERE MADE ON THREE ONTARIO NETWORKS OF DIFFERENT TYPES. IN THE SECOND, FIVE METHODS WERE USED TO ASSIGN TRAFFIC OVER THE ENTIRE TORONTO ROAD NETWORK WITH CAPACITY RESTRAINT ASSUMED. IT WAS CONCLUDED THAT THE MULTIPATH- ASSIGNMENT TECHNIQUE CAN BE USED TO ADVANTAGE FOR MOST NETWORK TYPES WITH OR WITHOUT CAPACITY RESTRAINT. WHEN RESTRAINT IS USED, AN ITERATIVE METHOD SHOULD BE ADOPTED TO PRODUCE REALISTIC LINK SPEEDS.

Media Info

  • Media Type: Print
  • Pagination: pp 136-138
  • Monograph Title: Transportation demand and analysis techniques
  • Serial:

Subject/Index Terms

Filing Info

  • Accession Number: 00228223
  • Record Type: Publication
  • Files: TRIS, TRB
  • Created Date: Feb 23 1973 12:00AM