Traffic Assignment

Traffic assignment Route assignment, route choice, or traffic assignment concerns the selection of routes (alternative called paths) between origins and destinations in transportation networks. It is the fourth step in the conventional transportation forecasting model, following trip generation, trip distribution, and mode choice. The zonal interchange analysis of trip distribution provides origin-destination trip tables. Mode choice analysis tells which travelers will use which mode. To determine facility needs and costs and benefits, we need to know the number of travelers on each route and link of the network (a route is simply a chain of links between an origin and destination). We need to undertake traffic (or trip) assignment. Suppose there is a network of highways and transit systems and a proposed addition. We first want to know the present pattern of traffic delay and then what would happen if the addition were made. Purpose of traffic assignment 1. To estimate the volume of traffic the links of the network and 2. Obtain aggregate net work measures. 3. To estimate inter zonal travel cost. 4. To analyze the travel pattern of each origin to destination(O-D)pair. 5. To identify congested links and to collect traffic data useful for the design of future junctions. 6. to provide necessary input and feedback to other planning tools. 7. to determine the deficiencies in the existing system 8. To estimate the volume of traffic on the links of the network and obtain aggregate network measures. 9. To estimate inter zonal travel cost. 10. To analyze the travel pattern of each origin to destination(O-D) pair. 11. To identify congested links and to collect traffic data useful for the design of future junctions. General principles THE GENERAL PROCESS OF TRAFFIC ASSIGNMENT Traffic Assignment in the Transportation Planning Process. As conceived today, a transportation planning study should be a cooperative, comprehensive, and continuing process. The principal objective of this transportation planning process is to determine the future form of the transportation network and the volume of vehicles or persons using any portion of this network. There are four phases in any planning process: (1) organizing for the study; (2) collecting and analyzing the data; (3) forecasting, formulating, testing, and evaluating the plan or plans; and (4) plan implementation. The first technical phase of the transportation planning process is the inventory of the existing conditions. Analysis of the data collected in these inventories provides the source information upon which the estimates of the future growth of the area are based. After estimates of the future travel have been made, the trips are then assigned to an assumed transportation network. The results are evaluated with reference to the desired level of service plus the social and economic consequences of the assumed system. Inevitably some revision will be necessary. The information obtained during this assignment is then used to modify the system, and another future travel assignment is made to the adjusted transportation network. This process is repeated until satisfactory results have been achieved. All assignment techniques are based on route selection. The choice of route is made on the basis of a number of criteria such as journey time, length, cost, comfort, convenience and safety. Journey time is often considered as the sole criterion since length and cost can be considered as function of time in most cases. The route selection Is made manually for small jobs but large jobs make use of an electronic computer for this purpose The traffic assignment procedure is based essentially on the selection by an electronic computer of a minimum impedance-path between zones. To accomplish this task, a description of the network is coded, key punched, and stored in the memory of the computer. After selecting the minimum impedance-path between zones, the computer proceeds to assign the trips to these routes. Traffic volumes are thus accumulated for each route section. For coding purposes, the route sections are considered to be the one-way part of a route lying between two intersections. They are referred to as "links." Intersections are points at which two or more route sections meet, allowing the possibility of a change in the travel direction. The intersections are referred to as "nodes." The centres of activity where trips are generated are also represented by nodes which are called "centroids." There is one centroid for each traffic assignment zone and each external station in the study area. There may be only four links connected to a node. All-or-nothing assignment All-or-nothing is often referred to as the minimum path algorithm. The minimum path, or tree, represents the minimum time path between two zone centroids and is assigned all of the traffic volume between the zones in question. As volumes and travel times increase, the results of this method become more unreliable. In this method the trips from any origin zone to any destination zone are loaded on to a single, minimum cost, path between them. The method is based on an assumption that the route adopted by the user will be the shortest time possible between the origin and destination. This model is unrealistic as only one path between every O-D pair is utilized even if there is another path with the same or nearly same travel cost. Also, traffic on links is assigned without consideration of whether or not there is adequate capacity or heavy congestion; travel time is a input and does not vary depending on the congestion on a link. How ever, this model may be reasonable in sparse and uncongested networks where there are few alternative routes and they have a large difference in travel cost. This model may also be used to identify the desired path : the path which h the drivers would like to travel in the absence of congestion. In fact, this model's most important practical application is that it acts as a building block for other types of assignment techniques. It has a limitation that it ignores the fact that link travel time is a function of link volume and when there is congestion or that multiple paths are used to carry traffic. Multiple route assignment technique All the road users will not be able to judge the minimum path for themselves. It may also happen that all road users may not have the same criteria for judging the shortest route. These limitations of the all or nothing approach are recognised in the multiple route assignment technique. The method consists of assigning the inter zonal flow to a series of routes, the proportion of total flow assigned to each being a function of the length of that route in relation to the shortest route. In an interesting approach suggested by burrell. It is assumed that a driver does not know the actual travel times, but that he associates with each link a suppose time. This suppose time is drawn at randowm from a distribution of times, having as its mean the actual link time. The driver is then assumed to select the route which minimises the sum of his supposed link times. Multiple route models have been found to yield more accurate assignment than al or nothing assignments. Capacity Restraint The inherent disadvantages of the above mentioned methods of not taking into consideration the actual capacity of the road network is over come in this method. The number of trips assigned to each link is compared with the capacity of the link to determine the extent to which link travel times have been increased by the additional volume placed on the formerly empty link. Using relationships between volume and travel time (or speed) it is possible to recalculate the new link travel time. A reassignment is then made based on these new values. The iteration process continues until a balance is achieved, such that the link travel time based on the loaded volume does not change with successive assignments. Many different capacity restraint equations have been developed and tested and are available for use. There are two basic characteristics common to capacity restraint models; (i) they are non-linear relationships and (ii) they use the volume-capacity ratio or v/c as a common factor. The underlying premise of a capacity restraint model is that the travel time on any link is related to the traffic volume on that link. This is analogous to the level of service (LOS) criterion, where LOS A corresponds to a low v/c and a higher vehicle speed. LOS E and the corresponding v/c = 1 represents capacity. Capacity restraint models assign traffic to possible routes in an iterative manner: 1. A portion of the total traffic volume is assigned to the link with the shortest travel time. 2. Travel times for all possible links are calculated again, since volumes have changed. 3. Another portion of the traffic volume remaining to be assigned is allocated to the link that now has the shortest travel time. 4. The travel time for all links are calculated and revised if changes result. 5. The process of incremental assignments, followed by calculation of revised shortest travel times, by link, continues until all trips have been assigned. The capacity restraint model used by FHWA is applied in an iterative manner. The adjusted link speed and/or its associated travel impedance is computed using the following capacity restraint function: 4 T=To[1+0.15(V/C) ] Where: T= balance travel time (at which traffic V can travel on a highway segment) To= free flow travel time: observed travel time (at practical capacity) times 0.87 V= assigned volume C = practical capacity Diversion curves For any trip from one zone to another zone there are usually several alternative routes, which can be chosen by the person making the trip.

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