Chapter 8 Travel Demand and Traffic Forecasting

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Chapter 8Travel Demand and Traffic Forecasting

• From Principles of Highway Engineering and
  Traffic Analysis
• Third Edition
• Fred Mannering, Walter Kilareski and Scott
  Washburn

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Mode Choice and Destination Choice

• Utility theory has been widely accepted to
  estimate modal and destination choices
• Utility theory assumes that travelers will select
  the mode and destination that provides the most
  utility (the largest value, benefit or
  effectiveness)
• Typically there is a portion of travelers
  utility functions that cannot be modeled, this is
  assumed to be randomly distributed

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Typical Utility Function
Vim total utility provided by mode/destination
alternative m to a traveler i bmk coefficient
estimated from traveler survey data for
mode/destination alternative m corresponding to
mode/destination or traveler characteristic
k zimk traveler or mode/destination
characteristic k (income, travel time of mode,
commercial floor space at destination, etc.) for
mode/destination alternative m for traveler
i eim unspecifiable portion of the utility of
mode/destination alternative m for traveler i
which is assumed to be random
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How to Determine What Mode/Destination will be
Chosen by Traveler i?

• McFadden developed the following logit model
  formulation which can be used

e base of the natural logarithm U is the
utility received by traveler i for selecting
mode/destination alternative m s notation for
available alternatives
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Logit Model Example

• Determine the probabilities of workers selecting
  various modes to travel to work
• DL drive alone
  SR auto shared-ride
  B bus
• Cost of driving auto 4.00/20 min travel time
• Cost of bus 50 cents/25 min travel time

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Final Step Route Choice

• Highway performance functions are used to model
  the relationship between travel time and traffic
  flow
• Two methods described in the text
• User equilibrium method individual travelers
  seek routes to minimize their personal travel
  time
• System optimization method seeks to minimize
  the total system travel time

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User Equilibrium Example

• Two routes connect and OD pair. A total of 4500
  veh travel the OD pair. Route 1 has 60mph speed
  limit and is 6 miles long route 2 has 45mph
  speed limit and is 3 miles long. With each 500
  veh added to route 1, travel time increases by 2
  min. Travel time on route 2 increases with the
  square of the of veh. Determine
  user-equilibrium travel times.

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User Equilibrium Continued

• Determine free-flow travel times, in min

x is traffic flow on route 1 and 2 in 1,000s of
vehicles per hour t is the average travel time on
route 1 and 2 in min.
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System Optimization

• Assumed that travelers will behave in such a way
  as to minimize the total system travel time

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System Optimization Example

• Using the same example

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System Optimization Problem Continued