Commission debates 25‑trip threshold, seasonality and study horizon in proposed Transportation Impact Study guidelines

Eastern Summit County planning staff presented draft Transportation Impact Study (TIS) guidelines at a March 7 work session, recommending a 25‑trip weekday AM or PM peak‑hour threshold that would trigger a transportation study and outlining scoping, trip‑generation, modeling and mitigation procedures.

Staff described the proposed threshold and said the Institute of Transportation Engineers (ITE) trip‑generation manual would be the principal source of trip‑generation rates used to estimate AM/PM peak‑hour trips for different land uses. Staff also emphasized an administrative caveat: the transportation planning director or county engineer could require a study outside the threshold where site‑specific conditions indicate it is warranted.

Multiple commissioners responded that the proposed 25‑trip threshold may be too high for Eastern Summit County. They argued resort and rural uses such as hotels, arenas, ski lifts and recreational facilities can create concentrated peak‑hour impacts that the threshold could miss, and they urged the guidelines be tailored to local conditions rather than relying on a single numeric cutoff. One commissioner said, “All these numbers are way too high. They would generate way more than 25 or more trips,” citing local car dependence and resort traffic patterns.

Staff explained how peak‑hour trip estimates are derived from ITE rates and that the purpose of a peak‑hour threshold is to capture projects that materially affect level of service or safety during the busiest hour of the day. Staff suggested that smaller subdivisions (for example five or six lots) typically do not change level of service and that requiring a study for every small project would be inefficient and costly.

Commissioners also questioned how studies should treat projects that are part of approved master plans; staff said studies already done at the master‑plan level could make separate TISs unnecessary in some cases but maintained the county should retain the ability to require additional study if local impacts warrant it. Commissioners discussed the study horizon, and staff proposed looking five years past assumed project completion for many projects while allowing longer horizons for very large, phased buildouts; commissioners asked for discretion to require longer horizons for multi‑decade projects like Cedar Crest.

The draft guidelines include new scoping items—public scoping, seasonal study time periods, trip distribution and phasing, level‑of‑service analysis, mitigation measures, safety analysis, and consideration of nonmotorized and transit connections. Commissioners asked staff to explicitly account for seasonality in nonmotorized mode assumptions since winter conditions reduce bike and pedestrian use in this part of the county.

Next steps: Staff will revise the draft guidelines to reflect commission feedback (threshold reconsideration, exceptions for master plans, phasing and study horizons, seasonality and context‑sensitive triggers) and present a revised draft to the commission before any code changes are prepared for council consideration.

Sources: Commission hearing transcript; staff presentation during the March 7 meeting.