Calvert County staff brief planning commissioners on traffic-impact analyses, signal timing and study scope

Calvert County Public Works staff gave the Planning Commission a technical briefing June 18 on how traffic impact analyses (TIAs) are prepared, how signal timing and level-of-service metrics are calculated, and how study areas are defined for proposed development.

The presentation outlined standard practice for traffic studies: field counts (usually by 15-minute turning-movement counts or video), peak‑hour selection, use of the Highway Capacity Manual methodologies and microsimulation tools such as Synchro/SimTraffic, and trip‑generation estimates derived from the Institute of Transportation Engineers (ITE) manuals. A county traffic analyst summarized: “Level of service … is sort of a grade that we give, like, intersections and segments of roadway and how well they're operating.”

Commissioners and members of the public pressed staff on several recurring concerns: whether the State Highway Administration’s (SHA) corridor signal timings are properly maintained and reoptimized, how emergency‑vehicle preemption affects coordinated systems, and whether the typical study-area radius (often one mile or to the first major intersections) captures regional commuting impacts. Staff said SHA uses coordinated timing for many Route 2/4/231 signals and that the county and SHA perform periodic reviews; they also confirmed emergency preemptions can temporarily “throw the whole system off” and require multiple cycles to reestablish coordination.

Why this matters: traffic studies determine whether mitigation is required for new development, and mitigation (from turn lanes to signal work) can materially affect project feasibility and concurrency with county policy. Commissioners noted repeated citizen complaints focused on choke points—Stokely Road and Dunkirk were singled out—and asked whether study areas and the county’s Adequate Public Facilities (APF) procedures were broad enough to capture the worst operational impacts. A commissioner proposed a policy change to require town‑center developments to include all town‑center intersections in TIAs; staff said the APFO ordinance and the director of public works retain discretion to expand study areas.

Key technical points and staff clarifications included: - Data collection: TIAs use recent turning‑movement counts (now often video) to identify the worst 60‑minute peak window; those counts are then grown to a future build year and combined with ITE trip rates to produce future no‑build and build conditions. - Level of service (LOS): LOS is calculated from average control delay (seconds per vehicle); the county typically treats LOS C as adequate on county roads, whereas SHA corridors often accept LOS D as adequate. - Signal timing: Signals may be isolated, coordinated (pre‑set plans by time of day), or adaptive. The county has coordinated corridors but not system‑wide adaptive control; staff said adaptive systems help non‑peak variability but may not materially improve peak‑period congestion. - Study-area scope: The standard study area is tuned to capture intersections directly affected by a site; staff said they sometimes expand the study area (as they did for a recent Prince Frederick case) when impacts plausibly extend to additional intersections. - Mitigation and responsibility: Major corridor fixes (for example Route 4 widening phases including the Stokely Road area) are SHA projects that require state funding; staff and commissioners agreed county staff will continue pushing SHA for design and construction funding but that it is difficult to require developers to pay for large state corridor projects.

Commissioners emphasized regional traffic growth—commuters traveling beyond local mile radii—and asked staff to consider how study scopes and APF rules reflect that reality. Staff reiterated that TIAs are estimates based on accepted practice, and that the director of public works can expand study areas or require additional analysis when warranted. The briefing closed with staff offering to share TIAs they receive (they are shareable on request) and reaffirming that modeling inputs, counts and assumptions are documented in each TIA.