Appendix C
2030 Washtenaw County Model
Travel Demand Model
Travel Demand Forecasting and traffic assignment in Washtenaw County is based on current and projected socio-economic data. These variables are projected based on existing and future land use plans from the municipalities and townships within the County.
Socio-Economic Data
WATS works with SEMCOG to project the following socio-economic variables by 5-year period over the life of the plan:
Population
Household
Employment total
Employment type
Workers by household
Workers by vehicle availability
Acres by land use
The land use concepts outlined and prescribed in local long range plans serve
as the basis for the forecasting of demand on the transportation system, not
just in terms of automobile trips but multiple modes of transportation. WATS
and SEMCOG staff work with local agencies to identify trends in development
and to discuss where future development is likely to occur and what type of
development it might be.
Over the past several decades, Washtenaw County has experience a large amount of suburban growth. The urban townships, Pittsfield, Scio, Ypsilanti and parts of Northfield have experienced a disproportionately high level of growth in population. This suburbanization of the County is expected to continue. This growth in population along with the inherent employment growth that comes with it greatly affects travel patterns within Washtenaw County as trips become longer both in distance and in time. Using projected socio-economic data, the WATS model helps to show the effect of this type of growth on the transportation system.
Trip Generation
The Washtenaw County model uses data from the 1994 household travel survey. This survey was used to develop the equations that are applied to represent the county's residents, workers, and visitors. The WATS model projects the following types of trips:
Home based work
Home based university
Home based shop
Home based other
Non-home based
This represents a significant advancement over the previous model the Agency used which only modeled three types of trips; home based work, home based non-work and non home based, did not have a transit network or a non-motorized component.
There are several areas in the County where using these generalized formulas do not produce accurate results because certain areas produce a disproportionately high number of one or more types of trips. These areas are called special generators and are adjusted to replicate the trips generated. In Washtenaw County, there are 11 traffic analysis zones that have "special generators" and therefore are adjusted. The 11 special generators used in the WATS travel model are:
University of Michigan Medical Center
St. Joseph Mercy Hospital
The Veterans Hospital and Nursing Home
Chelsea Community Hospital
Saline Community Hospital
Beyer Hospital
Briarwood Mall
Pioneer High School
Huron High School
Community High School
Ann Arbor Ice Cube/Wide World of Sports
Trip Distribution
Trips are distributed across
the highway and transit network based on the activities with the traffic analysis
zones as well as the proximity of the trip origins to major activity centers.
Trip generation and trip distribution work with the gravity model component
of the travel demand model. This is a factor within the model that produces
more trips close to activity centers and fewer trips, as populations are further
out from these activity centers.
Trip Assignment
The WATS model uses an equilibrium assignment. Equilibrium assignment uses multiple iterations to assign trips to the network in a very realistic way, using travel time as the most important factor in route rather than distance. With each iteration trips are reassigned based on the travel times caused by the first iteration. This makes it so when a route becomes congested and delays are likely to be seen (as a result of being above capacity) trips will be re-routed to a route that will arrive at the destination zone in the least amount of time, this process is repeated until all trips have been assigned the most efficient route based on time. Another component of this part of the model is the transit assignment. Using survey data, more equations are applied to trips which select out a given number of transit trips by using origin/destination, travel time, and costs that may be incurred as well as automobile availability.
Long Range Planning
In order to achieve an even more comprehensive congestion analysis WATS runs two different types of assignments a 24-hour assignment and a peak hour assignment. Using two different allows for areas that are designated as deficient to be looked at more carefully.
One of the most important components of travel demand modeling is understanding the trips that originate outside the county and end outside the county, simply "passing through". WATS uses survey data taken at these "external stations" (points where trips enter and exit the County) to estimate how many of these pass through trips occur and on what routes they travel. WATS plans to continually update these surveys with the help from consultants to ensure that these critical pass through trips are modeled correctly to show the most accurate representation of the demand placed on Washtenaw County's transportation system.