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102 I N N O VAT I O N S I N T R AV E L D E M A N D M O D E L I N G , V O L U M E 2 scale (lack of representational detail, specifically with and that will end their travel after 10:00 a.m. The regard to traffic queuing) or a microscopic scale (too affected route choices of vehicles will then of course computationally intensive to model an entire region). influence link flows in periods earlier than 10:00 a.m. The end of the simulation period, the cool-down period, after vehicles are no longer loaded on the network, is DTA MODEL SPECIFICATION therefore necessary as well. In the implementation, a 1-h warm-up period was The use of dynamic traffic models requires one to con- used. Three 1-h analysis periods (6:00 to 7:00, 7:00 to sider the specification of time much more carefully than 8:00, and 8:00 to 9:00 a.m.) were defined, for which is necessary in customary static network models. Time is flows were tabulated and compared with observed 1-h relevant in a number of contexts in a dynamic model. counts. Finally, a cool-down period sufficient to allow all First, demand is specified as the number of vehicles to vehicles to be simulated entirely from their origins to load on the network during a certain time period. The their destinations and therefore to exit the network was Atlanta regional model defined demand in a 4-h period used. Depending on how well converged the dynamic representing trips departing between 6:00 and 10:00 user-equilibrium solution was, the cool-down period a.m. Analysis periods should be defined over which could have been from 3 to 7 h. The simulation period results of the DTA model will be compiled and compared was therefore defined to start at time zero at 5:00 a.m. with observed data. These periods could be a single 1-h and end anywhere from noon to 5:00 p.m., depending period, several 1-h periods, several 15-min periods, and on how well vehicles were allocated to routes. The more so forth. converged results that were used to compare with The analysis period should almost certainly not align observed counts were typically based on 8 h of simula- with the demand period. If, for example, the demand tion time, with all vehicles exiting the network during period were 6:00 to 10:00 a.m. and the period of interest those 8 h. was 6:00 to 7:00 a.m., the DTA model would estimate Besides the demand and analysis periods, the DTA travel times on the network by simulating vehicles that assignment procedures use assignment intervals and link were entering the network starting at 6:00 a.m. The first aggregation intervals. An assignment interval is a length vehicles to load on the network would have no other of time when all vehicles traveling between a given origin vehicles to contend with, which in most urban areas is and destination and departing their origin during this unrealistic. If it is necessary to evaluate traffic simulated interval experience the same travel time at equilibrium. at 6:00 a.m., then some estimate of demand occurring When this state occurs over all assignment periods, the before 6:00 a.m., say from 5:00 to 6:00 a.m., should be DTA is in a state of dynamic user-equilibrium. At that determined and used to allow the DTA model to produce point, no vehicle has an incentive to follow a different realistic traffic levels at the times the intervals of interest route, and the DTA solution is stable. Assignment inter- occur. As is discussed later, getting realistic traffic at 6:00 vals were defined with lengths of 15 min. a.m. is easier than getting realistic flows at 8:00 and at Link aggregation intervals are the length of time over 9:00 a.m., as the network is continually loading. The which the simulated vehicle travel times on a link are demand period for trips departing their origins before averaged to yield a single link travel time for that aggre- the time interval of interest is often referred to as a warm- gation period. These average link travel times by aggre- up period. gation period are used in the time-dependent shortest The last vehicles to be loaded onto the network at the path (TDSP) calculations that are part of the DTA's end of the demand period also have an important effect dynamic user-equilibrium solution procedure. on other vehicles. For example, some number of vehicles will be loaded on the network just minutes before 10:00 a.m. It might be thought that those vehicles will only INPUT DATA REQUIREMENTS contribute to link flows after 10:00 a.m. and therefore that the simulation of vehicles need only occur between The input data for Vista include the Atlanta regional 5:00 and 10:00 a.m., giving 5 h of simulation time. How- highway network described as a link table and a node ever, vehicles entering the network near 10:00 a.m. will table, much as the network is defined for the regional have an impact on vehicles that started their travel ear- demand model. The link table contains node IDs at each lier. The vehicles entering at 10:00 a.m. may contribute end of the link, length, free speed, and link capacity to congestion on links at 10:00 a.m. or later. Had these information. The node table contains spatial coordinates vehicles not been simulated past 10:00 a.m., links could and a type to distinguish regular intersection nodes from be represented as having less congestion than they should centroid nodes. Vista also uses input tables to define the have, which could affect route choices made by vehicles location and operational characteristics of signalized that started their travel much earlier than 10:00 a.m., intersections in the network. Finally, Vista requires an