Summit County planners seek clearer rules for when traffic studies are required

Summit County planning staff on March 12 presented draft Transportation Impact Study (TIS) guidelines intended to make traffic-review expectations more consistent for developers and reviewers.

Brandon Brady, the county’s transportation planning deputy director, told the Snyderville Basin Planning Commission that the draft would set a clear trigger for when a study is needed and what it must include. "We require traffic impact studies for larger developments," he said, and the packet released to the commission proposes using a threshold of 25 or more new weekday AM or PM peak‑hour trips as the default trigger.

Why it matters: commissioners said the county needs predictable guidance so applications are not delayed by missing data or inconsistent consultant assumptions. Staff emphasized common local concerns — seasonal traffic in the Basin, mixed‑use reductions and transit proximity — and proposed standardized scoping meetings before applicants submit a study.

Key elements of the draft: the guidelines recommend using the Institute of Transportation Engineers (ITE) trip‑generation manual as a baseline, the county’s Summit‑Wasatch travel demand model for distribution, and a risk-based approach to forecasting. Staff suggested a default five‑year post‑completion forecast for most projects, with longer horizons for very large developments; Park City’s and other jurisdictions’ approaches were cited as comparators. Staff also proposed requirements to address:
- Project scoping and a required pre‑application meeting for consultants to discuss study area, modeling approach and special local factors;
- Study timing and seasonal adjustments so counts reflect peak winter or summer Basin conditions;
- Trip distribution using the county travel demand model or local data where appropriate;
- Traffic operations analysis and mitigation options (turn lanes, roundabouts, turn pockets, or Transportation Demand Management measures); and
- Nonmotorized and transit access, including a required section on pedestrian and bicycle connectivity and on transit stops within a quarter mile.

Commissioner questions focused on modeling assumptions and real-world behavior. One commissioner asked why single‑family detached and attached (condo) units receive different trip rates in ITE; Brady said the manual’s published rates are the basis but staff can research local adjustments. Another asked whether the county tests construction‑phase impacts; staff responded that construction access is typically handled by county engineering during building permit review, but it can be incorporated for large projects.

What commissioners want next: the commission asked staff to return with clarifications and supporting analysis: the sample calculations that show how the 25‑trip threshold was derived; the local calibration for mixed‑use reductions; explicit guidance for when sensitivity analyses are required; and language to make phased projects accountable to the long‑range transportation plan. Commissioners also encouraged staff to add a short staff memo accompanying big traffic reports that highlights likely weaknesses or unusual assumptions to help commissioners and the public interpret technical findings.

Next steps: Brady said staff will incorporate the comments from the Basin commission and from an Eastside planning commission review into a revised draft before pursuing code amendments to make the guidelines enforceable.