Benefit ($)/ Year
Average Annual Daily Trips (AADT) Reduced
Annualized Benefits per AADT Reduced
Next, the formula to convert program costs into an annualized basis is:
( ) ⎢ ⎣ ⎡ + 1 n i i (1+ i)n −1
⎥ ⎦ ⎤
Where: A = Annualized value, measured in dollars P = present value, measured in dollars i = annual interest rate n = length of the program, measured in years
Another benchmark measure that the model provides is a per passenger-trip benefit to cost ratio, defined as the ratio of annualized per passenger-trip benefits to per passenger- trip costs:
Annualized Benefits/ AADT Reduced
Annualized Costs/ AADT Reduced
As seen in the following example, this last measure provides a metric suitable for intra- comparison across different competing TDM alternatives and for inter-comparison, i.e., TDM strategy versus capacity expansion.
Applied Example: Transit Travel Time Improvement and Parking Cost Increase
A simple example provides some insight on how the constant elasticity of the demand model can be used to estimate impacts on travel behavior produced by changes in the generalized costs of two modes. We start by defining a base case scenario (absence of program strategy) by assuming a set of initial cost and travel time values for both SOV and transit: 17
= $ di = 3,000 vehicle round trips per day 6 . 0 0 c u r r e n t d a i l y S O V p a r k i n g c o s t 0 i P
17 Elasticity estimates are obtained from Litman . The study provides a summary of various elasticity estimates culled from several publications.