Laserfiche WebLink
is that more interactions (between different zones) take place when the cost of <br />interacting is less. As with the physics of gravitation between masses, it has been found <br />that many human interactions can be related to the distance or cost between interactors <br />using a negative exponential function. <br />In the Assignment portion of the simulation run, the distributed trips on the trip table <br />are assigned to possible paths between each zone. The assignment uses an algorithm <br />known as "equilibrium" assignment. This algorithm assigns traffic to each path <br />between each zone such that the travel time for each path between each path is <br />statistically equal. <br />Each model run consists of multiple feedback loops from trip assignment to trip <br />distribution. The skim matrix of travel time between zones is fed back after each <br />assignment into the trip distribution. The skim matrix is then averaged (weighted more <br />heavily towards the current iteration) between the previous iterations' skim matrix with <br />this iteration's skim matrix This method of skim matrix averaging and feedback loops <br />produces more stable results and more accurately represents the impact on travel <br />distributions from travel times. This allows the distribution to change as travel times <br />increase or change between zones. Trips will distribute to other zones which are easier <br />to reach or not as congested as reflected in the skim matrix. <br />This model also used Multi -Point Assignment (MPA). Traditionally all trips begin or <br />end at the zone centroid, a point in the center of the zone. In reality, trips begin at <br />driveways, parking lots, and other places in the zone. MPA allows the modeler to <br />define the access points for each zone. These were assigned equally for each connector, <br />except in cases where the access would be different. For speed of the model run, the <br />MPA function is turned off during the iterative distribution and assignment portion, <br />and then once the final trip matrices are established, MPA is turned back on for a highly <br />refined assignment. <br />The series of calibration simulation runs involves review of the assumptions used to <br />construct the model. In the distribution portion of the simulation, the exponents to the <br />distance function of the gravity model are examined. During the assignment portion of <br />the simulation, the assumptions for link speeds, capacities, and delay parameters are <br />studied. Between each run, different parameters are evaluated and necessary <br />adjustments made to the "rules" so that the desired results (i.e., calibration) are reached. <br />Kittitas County, Washington Transportation Model - May 2009 Page 7 <br />