6.0. HANDOUT SR 04-27-1998 - Laserfiche WebLink

Home Browse Search

ISeveral methods for assigning trips onto a network are available. Capacity restraint analysis is an iterative process whereby trips are assigned, the Inetwork is check for links whose assigned volumes exceed link capacity, travel times are adjusted upward on over-capacity links, and trips are The steps are reiterated until the network is balanced. This Ire-assigned. method shows how the network will be loaded after drivers adjust their travel behavior to compensate for congested areas, but it does not show how the network is loaded if every driver can take the most desired route. I All-or-nothing analysis, on the other hand, assigns every trip to its most desired path without considering segment capacities. Since the purpose of the Elk River Transportation Plan was to identify areas where congestion I could be anticipated in the future and provide solutions for these problem p p P areas so that the number of drivers forced by congestion onto alternate routes I could be minimized,we decided that an all-or-nothing analysis would be the most useful. It permits the planner to see where drivers want to travel, and to make changes to the system that encourage the driver to use that route. If Imore drivers can use their first choice routes through the network, fuel and travel time are saved. There is also a potential safety benefit in reduced congestion. A. Replication IThe first series of network assignments was a replication of the existing transportation system usage. The purpose behind the existing system was to arrive I at a combination of trip generation rates,trip distribution process and network link attributes that would accurately portray City travel patterns in an assignment. This combination would then be retained for travel demand forecasts. An initial I all-or-nothing assignment was made under the assumption that there is currently no location on the street network that drivers intentionally avoid because of a capacity deficiency. The assigned traffic volumes for each roadway segment were compared Iagainst actual field counted volume data. Segments whose assigned volumes differed to an unacceptable degree from actual volumes were noted. A series of screen lines I was used to check that there was an accurate representation of total volume traveling between one part of the City and another. (A screen line is a line drawn east-west or north-south across the entire City. Traffic volumes on all roadway segments crossing I the screenline are totaled and compared to the sum of existing volumes on the same segments.) Adjustments were made to trip generation rates,trip distribution and/or network link attributes, and a new assignment was made. This process was Icontinued until the existing system usage was satisfactorily replicated. B. Ultimate Forecast IThe trip generation rates and trip distribution process that were used in the final replication assignment were also used in the ultimate development assignment. Most Inetwork link attributes were also retained. However, there were a few notable exceptions. All county roads that had a 55 miles per hour speed in the replication I assignment were reduced to 45 miles per hour,because they would now be in fully developed areas. County Road 40 and 221st Avenue, both existing gravel roads, were assumed to be paved and straightened with speeds increased to 45 miles per ICTP-230.156 -17 230-156-80