TIA+, a more robust traffic impact analysis

This blog is about Traffic Impact Analyses and local Planning Boards. Local planning boards play an important role in calming traffic and can use traffic impact analyses/studies to do it. I’ll explain how and provide tools that your board can use.

Traffic impact analyses often lead to more traffic

Planning boards evaluate impacts that a development might have on neighbors, community and environment. They evaluate the impact on the sewer system, on the electrical grid, on the night sky, etc. The underlying premise is that new developments should not overtax the existing system, and if they so, contribute to expanding it. To use an analogy, certain developments have high electricity and water demands, which will overtax existing systems. It is common to require conservation techniques (low flow toilets, water meters, off-peak electrical use) to manage impacts.

To assist in theses evaluations, an applicant and/or a third party may prepare an impact analysis. In the case of transportation and mobility, this often takes the form of a traffic impact analysis (TIA).[1] Current practice typically only addresses the motor vehicle portion of the “impact”. The threshold is usually 100 vehicle trips in the peak hour – an arbitrary number. Should the threshold be met, the mitigation proposed is often increased roadway capacity, which leads to increased traffic - a vicious cycle.

TIAs do not typically explore alternate or non-traditional solutions to increased traffic such as shared driveways, sidewalk and path connections, shared parking, inter-parcel connections, etc.

Traffic impacts are not isolated

While the impact of any one development might indeed be small, each contributes to the overall cumulative impact of development. One of the shortcomings of individual TIAs is that they tend to result in a “last one in, pays” result in which the most recent development becomes responsible for the total cost of improvements, while only responsible for a portion of the impacts necessitating the improvements.

Changes to the roadway system should be evaluated and implement on a holistic network level. This is best performed by the City, not individual applicants.[2] Applicants should be required to contribute to their portion of any impact and mitigation, but the full cost of development should not be their burden alone.

Reforming traffic impact analyses

Many jurisdictions now require a multimodal approach that examines the existing and proposed impacts to walking and bicycling as well as to motor vehicle travel. In addition and where applicable, transit trips are also typically included in current studies.

The Institute of Transportation Engineers (ITE) has recognized this deficiency and has studied the idea of a more multimodal TIA process in recent years. In 2023, the ITE published its Recommended Practice document “Multimodal Transportation Impact Analysis for Site Development (MTIASD) - an ITE Recommended Practice” which contains general guidance for an expanded TIA process.[3]

The usefulness of the Level of Service (LOS) metric[4] traditionally employed in most TIAs has been called into question and is being reformed/replaced throughout the industry.[5] Jurisdictions are shifting from LOS to Vehicle Miles Traveled (VMT), which is seen as more reflective of induced travel/traffic.[6]

The traditional TIA process generally contributes to what is known as induced traffic or demand.[7] For example, Project A sits on a Road B. The project is expected to generate X number of trips. The roadway capacity is Y. If X causes the capacity to exceed Y, then an “improvement” is required, i.e. a lane is to be added. Paradoxically, as the additional lane makes it easier for cars to travel, this mitigation measure tends to exacerbate the issue as larger roads tend to encourage or induce more traffic.

TIAs typically predict vehicle trips. This can be a moving target, as it is in the interest of the applicant to minimize the impact of new trips. This explains why the vast number of TIAs find no significant impact. In that parking is a rough proxy for trips, a more useful approach may be to calculate the number of net new parking spaces on a site/building plan. Other metrics include proximity to a bus stop, walking distance to city hall, and net new square footage.

Planning board purview

Requirements for TIAs and thresholds thereof are within the purview of local Planning Boards in New York. Not only do TIAs help them during their consideration of land use applications, but also in their analysis of the potential environmental impacts that may result from proposed development when exercising their obligations pursuant to the State Environmental Quality Review Act (SEQRA).[8]

MODEL CRITERIA

Following is a six point set of criteria for Traffic Impact Analyses. Feel free to use and distribute.

1. Study area map

1.1. A map with streets, number of travel and turn lanes, sidewalks, bus stops, bus shelters, bike lanes, multi/shared-use paths, trails, crosswalks, and other elements of the transportation/mobility network.

1.2. The map is to include all transportation facilities (streets, sidewalks, pedestrian ramps, bike lanes, bus stops, etc.) within one-quarter mile of the site perimeter, and select facilities (as determined by [jurisdiction]) within one-half mile of the site perimeter.

2. [Jurisdiction] plans

2.1. A narrative describing how the proposed project will advance goals and specific recommendations found in [jurisdiction] plans, i.e. Comprehensive Plan, Mobility Plan, etc.

3. Parking and trips

3.1. Number of net new parking spaces. Applicants to minimize the number of spaces via sharing, on-street, land bank, etc.

3.2. Number of net new motor vehicle (MV) trips using best current practices: the Environmental Protection Agency’s (EPA) Mixed-Use Trip Generation Model,[9] the Institute of Transportation Professionals’ (ITE) Trip Generation Manual,[10] or a more local source (for example internal research). Applicants to offer strategies to minimize new external, peak-hour MV trips via transportation demand management (TDM).[11]

3.3. Number of net new walking trips.[12] Applicants to evaluate walking facilities in the study area and propose improvements (paths, crosswalks, sidewalks) to encourage walking.

3.4. Number of net new cycling trips. Applicants to evaluate cycling facilities in the study area and propose improvements to encourage cycling.

3.5. Existing and proposed travel times (walking, cycling, driving, transit) between the project site and destinations within the city (City Hall, library, local school, dog run, nearest grocery store, nearest park, etc.).

4. Traffic safety

4.1. A summary of recommendations found in safety analyses produced within the past five years.

4.2. Speed data for select streets, specifically maximum and 85th percentile speeds per hour over a representative 24-hour period. The intent is to show the highest speeds.

4.3. Three-year crash data for pedestrian, bicycle, and vehicle. A crash cost analysis using the KABCO scale.[13] An analysis of Economic Costs, QALY Costs, and Comprehensive Costs in most recent dollars. An analysis of crashes per number of lanes and crashes per average daily traffic (ADT). A comparison of crashes to other locations in the [jurisdiction].

4.4. Sight distance analyses at intersections, driveways, bus stops, and other points where people walking or cycling would be expected to cross the street.

4.5. A proposal for countermeasures to manage speeds and reduce crashes without limiting access for pedestrians and cyclists.[14]

5. Traffic volume

5.1. Vehicle, transit, truck, bicycle, and pedestrian volumes for representative time periods on select streets and paths.

5.2. Existing and proposed volume to capacity (v/c) ratio per hour on select streets, showing percentage increase/decrease.

5.3. A queue analysis to determine if the proposed project will routinely impede access to surrounding properties.

5.4. A proposed set of improvements that align with [jurisdiction] plans, and/or make more efficient use of existing infrastructure. Improvements should generally not lead to induced vehicle traffic (roadway widening, new turn lanes), nor should they fixate on localized peak hour traffic delays.

6. To ensure transparency, [jurisdiction] is to be copied on all correspondence during the review process between the applicant and other jurisdictions (state, county, transit agency, etc.).

Transportation impact analyses

Finally, let’s call these Transportation Impact Analyses. After all, the Institute of Traffic Engineers changed their name to Institute of Transportation Engineers in 1975.[15]