Chapter 4 - trip assignment

Question 1

What are the objectives of traffic assignment?

Answer 1

Primary:
•to obtain good aggregate network measures, e.g. total motorway flows, total revenue by bus service;
•to estimate zone-to-zone travel costs (times) for a given level of demand;
• identify heavily congested links.
• Secondary:
•to estimate the routes used between each O–D pair;
•to obtain turning movements for the design of future junctions.

Question 2

define user equilibrium

Answer 2

a user-equilibrium network is a network in which no user can improve his travel time (cost) by unilaterally
changing routes

Question 3

Three techniques of trip assignment:

Answer 3

• Minimum path (all-or-nothing) assignment
• Stochastic assignment
• Equilibrium assignment

Question 4

Minimum path steps:

Answer 4

1. Find shortest route between TAZs
2. Assign all trips to links comprising shortest route
3. Continue until all pairs have been assigned

Question 5

Minimum path advantages and disadvantages:

Answer 5

Advantages:
• Simple and inexpensive
• This is what users will do when there is no congestion (desire)
• Answers are easy to understand
Disadvantages:
• Unrealistic flow patterns

Question 6

Stochastic assignment:

Answer 6

randomness present.
• Each competing route in the in the path has a utility
• The utility will determine the probability of usage of the route
• a la – choice probabilities using logit model
•
This technique recognizes that different routes might be perceived to be equally
attractive to the user

Question 7

Equilibrium assignment:

Answer 7

This technique recognizes that ‘costs’ generally depend on the ‘flow levels’ during a trip
• Thus, the more congested a route is, the more expensive it is to use the route (N1 to Bellville)
• Not just the link travel time that is important, but also the capacity.
• Average speed decreases with average flow increase
• Average travel time increases as the volume-to-capacity ratio on
link increases
• Travel time depends on the loading, but the loading depends on the travel time – iterative process
LEARN THE GRAPHS

Question 8

Equilibrium assignment advantages and disadvantages:

Answer 8

Advantages:
1. More realistic … especially for peak flows
2. Trips are assigned to a variety of routes
3. A solution is always found
 Disadvantages:
• Not sure whether the flow patterns is optimal for user equilibrium (could be equilibrium but not optimal)
•All users “pay” the same cost pre route, but total travel time is not at a minimum.

Question 9

Issues in Public Transport Assignment:

Answer 9

1. Supply
   • Network difference. Includes
   • links (capacity of unit and frequency), terminals, stops
   • Other links influencing travel time
   •In-vehicles, wait, walk, transfer
   • Dedicated right-of-way / mixed traffic
2. Passengers
   • Movement of passengers – need to create walking and
   transfer links. Possibility of Park & Ride
3. Monetary Costs
   • Private car – associated with fuel consumption
   • Public transport – complex fare structures
   • by distance, time of day, individual characteristics (pensioner / school goer), flat fares,
   transfers etc
   4.The definition of Generalised costs
   • Can software handle all these factors and variables?
   5.The common lines problem
   • Parallel routes (for example express services) – allowing the choice of route being determined by the next service arrival.
   6.Frequency or Scheduled based route choice
   • Determining waiting time – use frequencies
   • High frequencies - easy to code
   • Low frequencies - easy to code
   • Irregular frequencies after the hour - hard to code
   • Coordination of low frequencies with other lines

Question 10

Stochastic disadvantages and advantages

Answer 10

Disadvantages:
Does not include capacity
Complexity in utility function
computational complexity
Hard to identify all routes
Advantages:
more realistic

Question 11

3 model differences:

Answer 11

minimum path - shortest
stochastic - user preference
equilibrium - congestion