Guide for Highway Capacity and Operations Analysis of Active Transportation and Demand Management Strategies
Appendix K: Long-Term Demand Effects
The demand adjustment model presumes that demand will respond to two factors related to weather, work zones and incidents; the added delay to pass through the work/incident zone and the proportion of travelers that are aware of the delay prior to entering the facility. The demand adjustment model is based on a travel time elasticity which suppresses total demand, a route diversion parameter, and a modifying factor based on the percentage of the drivers aware of the delay prior to entering the facility.
Equation 14
Where
V = The new demand (vph)
V0 = The original (current) demand (vph)T0 = The original (current) average door to door trip length for users of facility (min)
D = Delay, the change in travel time between the original and future condition (min).
β = Beta, travel time elasticity (default is -0.2)
γ = Gamma, route diversion parameter (see Table 46). TIMP = Traveler Information Market Penetration – proportion of drivers on facility that are fully and accurately aware of the delay prior to entering the facility (0-1). This will be a function of the timeliness, accuracy, quality, and detail of the information provided to the drivers on the facility.
For the average door to door trip length of facility users, use data from local household survey or regional travel demand model. Lacking that data, 22.9 minutes, the national average commute trip time, can be used as a default (FHWA, Summary of Travel Trends – 2009 National Household Travel Survey).
The selection of the traveler information market penetration (TIMP) will be sensitive to the degree to which the agency pushes weather, work and incident zone delay information to the web and mobile devices, and the degree to which drivers take advantage of it. The TIMP will also be sensitive to the timeliness, accuracy, quality and detail of the information provided to the traveler (for example alternate routes and/or comparative travel times). Obviously the greater the amount of advance warning that can be given, and the greater the number of media outlets will increase the market penetration rate of the work/incident zone information.
For the route diversion parameter (Gamma) select a value that results in reasonable estimates of route diversion for the selected facility location. This will be highly dependent on the availability of parallel routes with sufficient capacity, and therefore no default value can be provided. Table 46 provides a recommended Gamma diversion parameter according to the amount of extra delay that would be incurred taking the alternate route (as compared to current conditions on the freeway). For example, if the extra travel time of taking the alternate route is 5 minutes (when compared to current freeway travel time conditions), then a Gamma parameter value of -0.139 would ensure that 50% of the drivers will switch to the alternate route when delays on the freeway increase by 5 minutes over current levels.
Note: These Gamma diversion parameters will result in a 50:50 split between the freeway and the alternate route when the travel times on freeway and alternate route are equal.
The values in this table are computed using the following equation:
Equation 15
Where
γ = The Gamma diversion parameter to achieve the target diversion proportion (x) of traffic to alternate route when travel times on freeway and alternate route are identical.
x = the target proportion of freeway traffic diverted to the alternate route when travel times on the freeway and alternate route are equal.
D = The extra travel time taking the alternate route (minutes).
Employer-Based TDM Plans
The analyst selected values for the labor pool covered by participating employers and the expected reduction in SOV use by participating employees are used in the following equation to estimate the reduction in facility demand to be achieved by employer-based TDM plans.
Equation 16
Where
V = The new demand (vph)
V0 = The original (current) demand (vph)
PHBW = Proportion commuters in traffic stream (unit less). Default is 0.50
LPP = Proportion of labor pool covered by participating employers. (unit less)
SOVR = Expected proportional reduction in SOV use by employees at participating employers (unit less)
The demand effects of various employer-based TDM program elements can be estimated using sketch planning models such as TRIMMS (University of South Florida, Software to Assist TDM Programs).
For quick estimation purposes, Table 47 can be used to approximate the likely effect of an employer-based TDM program as a function of the percent of the county workforce represented by the employers participating in the TDM program. This table was constructed for employer-based TDM programs consisting of the following elements:
- Alternative or flexible work schedules;
- Telework;
- Rideshare program;
- Car sharing/bike sharing program;
- Guaranteed ride home program;
- Education Program on Commute Alternatives;
- Implementation of a commute trip reduction program;
- Participate in/create/sponsor a transportation management association;
- Employer Subsidies/Financial Incentives:
- Vanpool/carpool/parking-cash out financial incentives – between $2.50/trip and $5.00/trip; and
- Public transit financial incentive of $2/trip.
One additional option evaluated and shown in the table was adding a charge of $15/day for on-site employee auto parking (that was previously free) to the employer’s TDM program.
Note: Adapted from: Dowling Associates, I-580 Interregional Multimodal Corridor Study, San Joaquin Council of Governments, 2011.
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