CE 2710 Transportation Engineering The Four-Step Model Wesley Marshall, P.E. University of Connecticut March 2009

The Basic Transportation Model… Study Area Zones Attributes of Zones Socioeconomic Data Land Use Data “Cost” of Travel btw. Zones The Road Network Inputs Traffic Volume by Road Link Mode Splits Emissions Outputs

What’s in the Black Box? The Four-Step Model

The Four-Step Modeling Process… Trip Generation Trip Distribution Mode Choice Trip Assignment

Overview of the Four-Step Model

} } TRIP GENERATION Input: - No. of Housing Units - Office, Industrial SF Land Use Data Household Socioeconomic Data - HH Size - Income - No. of Cars Examples of HH socioeconomic data } Examples of land use data Iterative Process Output: Trip Ends by purpose - i.e. columns of trip productions and trip attractions TRIP DISTRIBUTION Input: Trip Ends by purpose Output: Trip Interchanges - a matrix of trips between each TAZ… also called a “trip table” MODE CHOICE Input: Trip Interchanges Output: Trip Table by Mode - a matrix of trips between each TAZ for each type of mode TRIP ASSIGNMENT Input: Trip Table by Mode Output: Daily Link Traffic Volumes - i.e. traffic flows on network, ridership on transit lines

P/A versus O/D Trip Production Home end of the trip (where the need to make a trip is generated) Trip Attraction Non-home end of the trip (where the need to make a trip is satisfied) Trip Origin # of trips that begin in a zone Trip Destination # of trips the end in a zone

Zone 1 Residential Zone 2 Non-Residential Zone 1 = Production & Origin Zone 1 = Production & Destination AM Peak Hour Trip PM Peak Hour Trip Zone 2 = Attraction & Destination Zone 2 Non-Residential Zone 2 = Attraction & Origin

Some General Problems with the Conventional Methodology Focus on vehicular traffic Better models typically have a transit component Typically forecasts huge increases in traffic Leads to engineers building bigger roads to accommodate “forecast” traffic Which leads to induced traffic and congestion… right back where we started when we needed the bigger roads in the first place

Some General Problems with the Conventional Methodology Pedestrians and bicyclists are rarely included Level of geography is difficult for non-motorized modes Network scale is insignificant Input variables are too limited

Four-Step Model Problem

4-Step Model Problem Given the zone data above and the tables/charts on the following page, Find the vehicle flows on the road network to the left using “all-or-nothing” traffic assignment. This will require completing all four steps of the transportation planning process…

4-Step Model Problem

Basic Step-By-Step Process Trip Generation Using the socioeconomic & land use data along with the cross-classification tables, come up with the number of trip productions & attractions for each zone… Trip Distribution Create trip interchange table based upon “friction” between zones using the travel time matrix, the F-Factor graph, and the Gravity Model… Tij = Pi Aj Fij ΣAjFij (Productions)(Attractions)(Friction Factor) Sum of the (Attractions x Friction Factors) of the Zones = Gravity Model Mode Choice Create a separate trip interchange table for each mode based upon, in this case, the relationship of land use household density the percentage of people walking & biking… Trip Assignment Distribute the vehicle trips to the street network using “all-or-nothing” traffic assignment by assuming that all trips are accommodated on the shortest possible path between zones…

1 2 3 4 Travel Time = 2 Travel Time = 5 Travel Time = 6

586 + 97 = 683 1 Travel Time = 2 2 787 + 159 + 40 = 986 159 + 658 = 817 Travel Time = 5 95 40 + 66 = 106 Travel Time = 6 97 + 215 = 312 Travel Time = 2 527 32 3 4 Travel Time = 5 131

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