What is modeling?
Many of the transportation planning activities performed by SRTC require Travel Demand Modeling. Modeling is a method of forecasting how society will use the transportation system now and in the future. Modeling involves the use of computer software to represent how travel choices are made. Travel demand occurs as a result of thousands of individual travelers making individual decisions on how, where and when to travel.
Modeling analysis explores various modes of travel including bus, auto and carpool. Modeling considers the demand on the transportation system and assists the public, community leaders, and partner agencies in making transportation decisions and investments.
The model contains inventories of existing roadway facilities and of all housing, shopping and employment in the area. Using the model, transportation planners can estimate future (as far out as 20 years) traffic volumes without the costs of building inappropriate roadways or waiting for traffic congestion to severely impact travelers.
SRTC is federally certified to conduct regional transportation modeling, and provides transportation modeling services to public jurisdictions and agencies in Spokane County. In house, models are used extensively for developing the Metropolitan Transportation Plan (MTP), a 20 year ‘blueprint’ for transportation planning SRTC is required to update every four years.
Transportation Modeling Programs used by SRTC
SRTC does several kinds of modeling, using different software, including:
Travel demand model – VISUM software
- Used to predict transportation measures such as traffic volumes and volume-to-capacity ratios on the regional transportation system.
- The model’s land use data comes from a variety of sources: WA State Employment Security Department, Department of Revenue, Department of Health, Census, and building permits.
- Travel demand results are often the input for intersection and air quality models.
VISUM is implemented in Spokane as a traditional “4-step” model:
- Trip Generation (land-use & economic factors determine how many trips people will make)
- Trip Distribution (determines where trips start and end)
- Mode-choice (determination of how you get to your destination; e.g., bus, walk, auto, etc.)
- Trip Assignment (determination of which path you take from your trip origin to your trip destination)
Microsimulation modeling – VISSIM software
Microsimulation means that a much higher level of detail is used in modeling to accurately simulate urban and highway traffic. This program can simulate several different kinds of traffic, including vehicles (cars, busses and trucks), public transportation (busses), bicyclists, pedestrians and rickshaws. In Spokane, it is currently used to model ?? VISSIM can be used to simulate different proposed traffic scenarios.
VISSIM provides a wide range of user-defined parameters such as number of vehicles, average speed, travel time, and delay time.
Intersection Modeling – HCS and Synchro software
The programs HCS and Synchro are typically used to model activity at intersections, such as level of service (LOS) during a specific time period, usually the morning or evening commute time.
Input used to utilize these programs includes traffic volumes, signal timing, and lane configurations at intersections. LOS is an indicator of how well traffic is moving compared to its designed capacity. Classic LOS is identified in terms of average delay per vehicle in seconds. LOS ranges from “A” to “F”, with “A” having the least amount of delay.
LOS in Terms of Average Delay per Vehicle
Level of Service Average Delay per Vehicle
A < 10
B 10 to 20
C 20 to 35
D 35 to 55
E 55 to 80
F > 80
Air quality modeling – CAL3QHC and MOVES
Spokane is designated as an attainment area for Carbon Monoxide (CO) and PM10,, operating under maintenance plans since August of 2005 (for more information on Air Quality, see the Air Quality page of this website). On-road mobile sources such as vehicles produce an estimated 67% of the total CO emissions for the Spokane area. As a result, SRTC is charged with Air Quality modeling to ensure that future transportation choices will not impact our air quality in a negative way.
CAL3QHC is a modeling program that predicts air quality impacts of carbon monoxide (CO), nitrogen dioxide (NO2), particulate matter (PM), and other inert pollutant concentrations from moving and idling motor vehicles at or alongside roadways and roadway intersections.
It is used for ‘Hot-Spot Analysis;’ determining the air quality levels in specific locations. Inputs used with this program include emission factors, roadway geometries, signal timing, weather conditions, site-specific conditions, current traffic volumes, and forecast traffic volumes.
MOVES (MOtor Vehicle Emission Simulator) is a modeling system that estimates emissions for mobile sources covering a broad range of pollutants. MOVES currently estimates emissions from cars, trucks & motorcycles.