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Transportation Model

The SANDAG transportation model provides a systematic analytical platform so that different alternatives and inputs can be evaluated in an iterative and controlled environment.

SANDAG Transportation Modeling

SANDAG deals with many complex mobility issues facing the San Diego region, including the development of a long-range Regional Transportation Plan (RTP). Transportation and land use models perform a very basic yet vital set of functions. Models are the principal tools used for alternatives analysis, and they provide planners and decision makers with information to help them equitably allocate scarce resources. The SANDAG transportation model provides a systematic analytical platform so that different alternatives and inputs can be evaluated in an iterative and controlled environment.

For the 2050 RTP, SANDAG uses an enhanced four-step transportation model. Four-step models have been the standard in transportation modeling since the late 1950s, and they are used by nearly every MPO in the United States for the development of transportation plans, corridor studies, Federal Transit Administration New Starts proposals, and air quality analyses. The estimates of regional transportation related emissions analyses meet the requirements established in the Transportation Conformity Rule, 40 CFR Sections 93.122(b) and 93.122(c). These requirements relate to the procedures to determine regional transportation-related emissions, including the use of network-based travel models, methods to estimate traffic speeds and delays, and the estimation of vehicle miles of travel.

The four major steps of the transportation model include:

  • trip generation;
  • trip distribution;
  • mode choice; and
  • network assignment (highway and transit).

Trip Generation

A trip generation analysis is the first step in the transportation modeling process. Trip generation estimates the average weekday number of trip productions and attractions, or trip ends, in the region based on land use and demographic information from the regional growth forecast. Over a 24-hour period, roughly the same number of trips will originate in a zone as are destined there. However, residential zones will generate primarily trip productions while nonresidential zones will generate primarily trip attractions. The production/attraction distinction is important for the trip distribution model discussed in the next section.

Trip Distribution

After trip generation, trip distribution allocates and balances trip productions and attractions through a gravity approach based on trip end density and location. Trip distribution considers the distance between a trip ends that is based on the assumed highway and public transportation networks that are input for any given future year. The model is designed to modify trip patterns in response to new land use developments and transportation facility changes. For example, the opening of a new shopping center would shift trips from other nearby shopping areas to the new development. Another example would be the introduction of mixed-use development. In this case the model would yield shorter trip lengths by recognizing the increased opportunity for interaction between residential and commercial areas in the development.

Mode Choice

At this point in the modeling process, total person-trip movements between zones are split into different forms of transportation by highway, transit, and non-motorized modes (bicycling and walking). The mode choice step selects the most likely form of transportation for each trip, based on access, traveler’s income, trip purpose, parking costs, fuel price, transit fares, travel time, and other time and pricing parameters.

Highway and Transit Assignment

During network assignment, the model places each trip on the most efficient auto, transit, or non-motorized path based on the mode of transportation that was chosen earlier. Highway assignment produces traffic-volume estimates for all roadway segments in the system. These traffic volumes are an important input to emissions modeling. Similarly, transit trips are assigned to transit routes and segments.

Model Iteration and Equilibrium

Once these four steps are completed for the millions of trips in the region on an average weekday, the SANDAG model iterates the trip distribution and mode choice step and runs through traffic assignment again based on levels of congestion measured in the previous iteration. The iterations continue until all trips are assigned the most efficient path for their mode. Each step is sensitive to an extensive set of inputs used to prepare a model scenario.

Modeling Software

TransCAD 5.0 is the transportation planning computer package used by SANDAG to provide a framework for performing much of the computer processing involved with modeling, and it is used for the trip distribution and assignment steps. ArcInfo, a Geographic Information System (GIS), is used extensively in the modeling process as well to maintain, manipulate, and display transportation, land use, and demographic data. SANDAG has written numerous customized programs that provide a linkage between TransCAD and ArcInfo. Other custom programs perform some modeling functions, such as trip generation and mode choice.

Data Sources

A number of data files and surveys are used to calibrate the transportation models. There are four major inputs to the transportation models:

  • Growth forecast inputs used to describe existing and planned land use patterns and demographic characteristics (described in Technical Appendix 2);
  • Survey information;
  • Highway networks used to describe existing roadway facilities and planned improvements to the roadway system; and
  • Transit networks used to describe existing and planned public transit service.


The transportation models make use of survey data to establish relationships between input variables and model-estimated results. For example, trip generation rates are applied to dwelling units from the growth forecasting process to determine the number of trips generated from residential areas. Data collection is costly and time consuming, so surveys are conducted relatively infrequently. This normally does not create a problem since underlying model relationships are relatively stable over time. Surveys used include:

  • 1995 and 2006 Travel Behavior Survey
  • 2001 Caltrans Statewide Travel Survey
  • 2001-2003 San Diego Regional Transit Survey
  • External Trip Surveys (2006 Interregional Travel Behavior Survey)
  • Traffic Generation Studies
  • 1991 San Diego Visitor Survey
  • 2000 Census Transportation Planning Package
  • 2010 Gateway Forecast
  • 2002 Freight Analysis Framework
  • 2010 San Diego Parking Inventory Study

Highway Networks

The regional highway networks in the 2050 RTP include all roads classified by local jurisdictions in their general plan circulation elements. These roads include freeways, expressways, and the Regional Arterial System (RAS). The RAS consists of all conventional state highways, prime arterials, and selected major streets. In addition, some local streets are included in the networks for connectivity between zones.

The route improvements and additions in the 2050 RTP are developed to provide adequate travel service that is compatible with adopted regional policies for land use and population growth. All regionally significant projects are included in the quantitative emissions analysis. These include all state highways, all proposed national highway system routes, all regionally significant arterials, and all “other principal arterials” functionally classified by the Federal Highway Administration.

The networks also account for programs intended to improve the operation of the highway system, including High Occupancy Vehicle (HOV) lanes, Managed Lanes, and ramp metering. Existing and proposed toll facilities also are modeled to reflect time, cost, and capacity effects of these facilities. The State Route (SR) 125 South, SR 11, SR 241, and additional lanes on Interstate 15 (I-15) north of SR 78 as well as additional lanes on I-5 north of Vandegrift Boulevard are modeled toll facilities included in the Revenue Constrained Plan for the San Diego region.

In addition, several managed/HOV lanes are included in the Revenue Constrained Plan. Facilities with proposed Managed Lanes include I-5, I-15, and I-805; and SR-52, SR-78, and SR-94. Managed Lanes are defined as reversible HOV routes and HOV routes with two or more lanes in the peak direction. Additionally, one-lane HOV facilities that operate as two-person carpool lanes in the earlier years of the plan transition to three-plus person managed lanes after 2035. It is assumed that the excess capacity not used by carpools and transit on these facilities would be managed, so that single-occupant vehicles could use these lanes under a pricing mechanism. Traffic flows would be managed so that the facility would operate at service level D or better.

SANDAG maintains a master highway network from which a specific-year network between the years 2008 (the 2050 Regional Growth Forecast base year) and 2050 can be built. Networks were built and verified for air quality conformity and SB 375 analyses of the 2050 RTP and EIR.

A list of the major highway and near-term regional arterial projects included in the analysis, along with information on phasing their implementation, is included in tables A.2 and A.4, located in Appendix A. Locally funded, regionally significant projects also have been included in the analysis. These projects are funded with TransNet funds, a 20-year, half-cent local sales tax for transportation that expired in 2008; TransNet Extension funds, a 40-year, half-cent local sales tax extension approved by voters in 2004 that expires in 2048; and other local revenue sources.

Transit Networks

SANDAG also maintains transit network datasets for existing and proposed transit systems. Most transit routes run over the same streets, freeways, HOV lanes, and ramps used in the highway networks. As a result, the only additional facilities that are added to the transportation coverage for transit modeling purposes are:

  • Trolley and commuter rail lines
  • Streets used by buses that are not part of local general plan circulation elements
  • Transit guideways

BRT service will have stations similar to commuter rail and light rail, and operating characteristics midway between rail and bus service. BRT service will be provided by advanced design buses operating on HOV lanes, some grade-separated transit ways, and surface streets with priority transit systems. Rapid Bus service would also utilize advanced design buses but they operate primarily along arterials. They would use queue jumpers and traffic signal priority measures to keep them operating at higher speeds.

Once TransCAD transit networks have been built, TransCAD finds minimum time paths between TAPs. The following four sets of paths are created for modes:

  • A.M. Peak-period local bus
  • A.M. Peak-period premium service
  • Midday local bus
  • Midday premium service

Bus speeds assumed in the transit networks are derived from modeled highway speeds and reflect the effects of congestion. Regional and express transit routes on surface streets are assumed to operate faster than automobiles in congestion due to priority transit treatments. Higher bus speeds may result for transit vehicles operating on highways with HOV lanes and HOV bypass lanes at ramp meters, compared with those routes that operate on highways where these facilities do not exist. In addition to transit travel times, transit fares are required as input to the mode choice model. TransCAD procedures replicate the San Diego region’s complicated fare policies which differ among:

  • Buses, which collect a flat fare of between $1 and $4, depending on the type of service;
  • Trolleys, which charge $2.50 for all trips;
  • SPRINTER, which charges $2;
  • Street-Cars, which are proposed to charge $2.25;
  • Commuter rail (COASTER), which has a zone-based fare of between $4 and $5.50;
  • Proposed regional BRT routes, which are assumed to charge $3 or $5 (for express BRT); and
  • Proposed Rapid Bus routes, which are assumed to charge $2.25.

Fares are expressed in 1999 dollars (consistent with household incomes from the 2050 Regional Growth Forecast) and are assumed to remain constant in inflation adjusted dollars over the forecast period.

Near-term transit route changes are drawn from the Regional Short-Range Transit Plan, which was produced in cooperation with the region’s transit agencies. Longer-range improvements are proposed as a part of the RTP development and other transit corridor studies. In addition to federal and state funded projects, locally funded transit projects that are regionally significant have been included in the air quality conformity analysis of the 2050 RTP. These transit projects also are funded with TransNet funds or other local revenue sources. Once network coding is completed, the transportation models are run for the applicable scenarios. A list of major regional transit projects included in the draft air quality conformity analysis, as well as information on phasing their implementation, is included in table A.3, located in Appendix A.