Traffic study trigger map

How the map reads a state

A traffic impact study, called a traffic impact analysis (TIA) in some states, is the study a state transportation department asks for when a development adds enough trips to its highways. The map shades a state when our research verified the department's trigger from its own manual, guideline or rule, and it lists every trigger row it holds for that state, with the unit and the condition the department writes. Arizona's TGP 240 calls for a traffic impact analysis at 100 or more trips in any one hour, Category I from 100 to under 500 and Category II at 500 or more, and a traffic statement under 100 that includes turn-lane analysis per TGP 245/430 ^[1]. All three rows sit under Arizona on the map. A state with no verified trigger reads not verified in our research.

The title of each shaded state reads its trigger rows in order, and the panel beside the map adds the document each one comes from. The rows are never reduced to one number for a state, because the units differ: some departments count trips in the peak hour, some in any one hour, some daily trips, and some only new trips. The filter picks out the states with a trigger counted in a peak hour or any one hour, and the states with a daily trigger, read from each row's own unit. Which trips a site adds is a question for the car wash stacking calculator and the gas station site calculator, which estimate them and set them beside these rows.

Daily and hourly triggers

Several departments count daily trips. North Carolina sets its threshold at 3,000 or more daily trips, allows no trip reductions when testing it, and below it leaves the call to the District Engineer ^[2]. Minnesota recommends a study at 250 or more peak-hour trips or 2,500 new daily trips ^[3]. Pennsylvania's policy dated 6/4/2025 sets Condition 1 at 3,000 or more trips per day and Condition 2 at 150 or more entering or exiting in any one hour ^[4]. The North Carolina and Minnesota state pages set these studies inside the wider approval path.

Others count a single hour. South Dakota calls for a study at 100 or more trips in the adjacent highway's peak hour or the development's peak hour ^[5]. New Mexico requires a site threshold analysis for all sites accessing a state highway, and a traffic impact analysis when that analysis shows 100 or more peak-hour total trips, under 18.31.6.16 NMAC ^[6]. Ohio calls for a full TIS above 200 peak-hour trips, a Traffic Analysis at 60 to 200, and a turn-lane warrant submission only at 10 to 59 ^[7]. The Ohio state page covers the access permit that brings a site under that manual.

Tiers, levels and rezonings

Tennessee's Traffic Design Manual sets four levels. Level 1 applies at 50 to 99 peak-hour or 250 to 499 daily trips. Level 2 applies at 100 to 249 new peak-hour or 500 to 2,999 new daily trips, with a 1/4-mile study area. Level 3 applies at 250 to 399 peak or 3,000 to 5,999 daily trips, with a 1/2 mile study area, and Level 4 at 400 or more peak or 6,000 or more daily trips, with a 3/4 mile study area ^[8]. The map lists each of the eight rows under Tennessee, as the Tennessee page's approval path meets them.

Virginia splits its rows between a rule and a guideline. Its administrative guidelines set study tiers of 100 to 499 and 500 or more peak-hour trips ^[9]. Its regulation, 24VAC30-155-40, amended effective 12/31/2011, calls for a rezoning submission at more than 5,000 vehicle trips per day at a state highway connection, or more than 400 per day for residential rezonings exceeding the highway's daily volume ^[10]. The Virginia state page sets those submissions beside the site plan review clock.

Texas and the district rows

Texas has no single statewide trip threshold: TxDOT's Access Management Manual says its scenarios are not meant to be used as thresholds ^[11]. The map shades Texas for that verified statement and lists the TxDOT Houston District's own rows under the district's name: a major study at 150 or more peak-hour or 1,500 or more daily trips ^[12]. A district row binds only inside that district, which is why the map never lifts it into a statewide figure. The Texas page covers the frontage road and state highway access that brings these rows into play.

The documents behind the rows

Each row carries the edition of the document it comes from, because a department can change a threshold with a manual revision. Arizona's rows come from TGP 240, dated Aug. 2021 ^[1]. Tennessee's levels come from Table 2.1 of the Traffic Design Manual of Aug. 2018 ^[8]. North Carolina's threshold sits in the TIA Checklist Instructions of 7-21-2017, under its Policy on Street and Driveway Access ^[2]. Ohio's tiers come from the State Highway Access Management Manual as revised July 2026 ^[7]. Minnesota's recommendation sits in Chapter 5 of its Access Management Manual ^[3], and South Dakota's trigger in Chapter 17 of its Road Design Manual ^[5]. Virginia's rezoning rule also requires the ITE 8th Ed. for the trip count ^[10].