Urban Transportation Planning�Module 7: Traffic Assignment Techniques

Traffic Assignment Techniques

1. Urban Transportation Planning Module 7: Traffic Assignment Techniques (Diversion Curves; Basic Elements of Transport Networks, Coding, Route Properties, Path Building Criteria, Skimming Tree, All-or-Nothing Assignment, Capacity Restraint Techniques, Reallocation of Assigned Volumes, Equilibrium Assignment, Multipath Assignment Technique) CSRK Prasad

2. All or nothing assignment Capacity restraint assignment Equilibrium assignment Multipath assignment….. Traffic Assignment – Analytical Methods

3. All or Nothing Assignment The path taken by vehicles travelling from zone of origin to zone of destination will be one with least travel resistance. Procedure is as follows: The determination of minimum path time from each zone with originating traffic to all other zones (Build Trees) The assignment of all traffic flows from each zone to every other zone by the appropriate minimum path, and the aggregation of total flows on each link in the defined network

4. All or Nothing Assignment Assumptions Link costs are fixed; i.e. no congestion All drivers think alike Every driver from i to j chooses the same route All drivers are assigned to that route and none to others These assumptions are reasonable in sparse and uncongested networks with few alternatives routes. The all or nothing approach really gives a desire line i.e . what drivers would do if all choices were available to them and if congestion was not an influence.

5. All or Nothing Assignment Drawbacks The technique takes no account of increasing congestion associated with increased volumes and assign too many vehicles to the better routes Some differences in journey times by different routes between the same origin and destination can bring about unrealistic journey paths

6. Capacity Restraint Assignment An alternative method of dealing with overloaded links in the network The input required is similar in many ways to that required by all or nothing assignment Practical Capacity of each link of the network is required as well as journey time

7. Capacity Restraint Assignment First set of minimum paths between zone centroids is determined like in AON method Traffic from each zone of origin to to each zone of destination is then assigned to the network The loads on each link are compared with the practical capacity of that link If a link is found to be overloaded then a new journey time which makes allowance for the effect of congestion or excessive traffic flows on speed, is calculated for that link. An iterative procedure, in which loaded link information is used as a feedback to the tree building process, is adopted.

8. Capacity Restraint Assignment Wayne State Assignment Method TRC Trip Assignment Model US FHA Method  

9. Travel Time – Flow Relationship

10. A typical Monotonically Increasing Capacity Function

11. Capacity Restraint Assignment Calculate the minimum time path trees, by using initial times as free flow travel times, Assign the traffic as an All or Nothing Assignment basis & determine link flows Calculate the new link times using the following link capacity function; BPR. is free flow travel time = 0.87x journey time at practical capacity  

12. Capacity Restraint Assignment To calculate the new minimum path trees, for each link the following revised times are computed: Return to step 1 using as link times to develop the minimum paths. Then repeat steps 2 to 4. Finally, the volume assigned is the average volumes of the flow as obtained in the previous steps.  

13. Capacity Restraint Assignment Zone to zone traffic volume can be assigned 100% , or In stages: 40% + 30% + 30% or 40% + 30% + 20% + 10% or 50% + 30% + 20 %, etc.

14. Readings Bruton, M. J., An Introduction to Transportation Planning (The Living Environment), UCL Press, London, UK, 2000. Hutchinson, B.G., Principles of Urban Transport Systems Planning, McGraw Hill, 1974. 8/1/2019 CSRK Prasad NIT Warangal 14

15. Contact: [email protected] [email protected] 0870-2462117 (O) 9440347348 (M) 2019-08-01 15 CSRK Prasad