Traffic Impact Analysis Standards

PURPOSE

These standards present policies, detailed procedures, and minimum analysis parameters and criteria for uniform development of traffic impact analysis studies, which enable preparers to present findings and recommendations in a systematic manner, based on sound engineering principles, consistent with the expectations and practice of Mohave County.

TRAFFIC IMPACT ANALYSIS STANDARDS

A traffic impact analysis (TIA) shall be required for all new land developments or additions/modifications to existing land developments expected to generate 75 or greater new peak hour trips per day based on highest peak hour of generator estimates derived from data contained in the Institute of Transportation Engineers’ Trip Generation, latest edition.

A traffic impact statement (TIS) shall be required for all new land developments or additions/modifications to existing land developments expected to generate 10 to 74 new peak hour trips per day based on highest peak hour of generator estimates derived from data contained in the Institute of Transportation Engineers’ Trip Generation, latest edition.

Mohave County arterial- and collector-classified roadways operating at Level of Service C or better under the condition – future traffic without development in horizon year – shall be mitigated to a Level of Service C. Mohave County local-classified roadways operating at Level of Service D or better under the condition – future traffic without development in horizon year – shall be mitigated to a Level of Service D.

All TIA recommended and County approved road and/or traffic control improvements shall be designed and constructed in accordance with or exceed design policy set forth in Mohave County Standard Specifications and Details and guidelines herein. The County may consider and approve exceptions on basis of engineering judgment supported by relevant engineering data and calculations.

TRAFFIC IMPACT STATEMENT GUIDELINES

1. Estimate total trip generation.

Reference data in Institute of Transportation Engineers (ITE) Trip Generation, latest edition.
Use highest (AM or PM) peak hour of generator; many Mohave County roads exhibit peak in traffic volume during midday hours.
Evaluate use of rate versus equation; consider and compare size of subject development against generator sizes in ITE published studies.
Local and/or brand-specific trip generation studies may be acceptable with discussion and presentation of individual location characteristics and study data.

2. Estimate daily site-generated truck traffic, FHWA Vehicle Class 5 and above (e.g., total single-unit and semi tractor-trailer trucks).

3. Present directional distribution.

For developments producing trips (e.g., residential), consider effect of area destinations (location and relative weight of attractiveness) from the site on directional distribution.
For developments attracting trips (e.g., commercial), consider effect of area origins (location and relative weight of attractiveness) of trips to the site on directional distribution.
Existing traffic volume distribution may – but not always – approximate directional distribution.

4. Present trip assignment to/from all proposed site access driveways.

Turning movements to/from proposed site access driveways must include site generated passby trips.

5. Document location and configuration of all proposed site access driveways adjacent County roads.

6. Calculate total crash rate for each County road adjacent a proposed site access driveway(s).

Obtain the 5-year crash history for the County road(s) from Mohave County Public Works; document total crashes and fatal crashes.
Obtain the average daily traffic for the County road(s) from (1) ADOT Transportation Data Management System or (2) Mohave County Public Works.
Calculate total crash rate as crashes per million vehicle miles traveled.
Obtain the latest published State total crash rate from the latest ADOT Arizona Motor Vehicle Crash Facts; take total crashes divided by millions of vehicle miles traveled as published (typically in Table 1-1).
Compare State total crash rate to total crash rate for each County road evaluated.

7. Analyze proximity of proposed site access driveway(s) to other intersections, including intersections of other site access driveways.

Evaluate and confirm the functional area of the proposed site access driveway(s) are does not overlap the functional area of any adjacent intersection on the same road.
Intersection functional area means the distance traveled by vehicles (1) while decelerating on approach to an intersection to stop for traffic control or permissive left-turn and (2) while accelerating away from an intersection after stop or turning maneuver.

8. Determine need for auxiliary lanes (left-turn and right-turn lanes) on roads adjacent proposed site access driveway(s) through Section 245 of the ADOT published Traffic Engineering Guidelines and Processes, latest edition.

If the State total crash rate exceeds the total crash rate for the County road evaluated, then apply the Section 245 criteria for right-turn traffic volume and left-turn traffic volume at 80% of published value.

9. For any recommended new auxiliary lanes, determine minimum turn lane length based on through Section 430 of the ADOT published Traffic Engineering Guidelines and Processes, latest edition.

TRAFFIC IMPACT ANALYSIS GUIDELINES

1. Complete Traffic Impact Statement Guidelines.

2. Identify County intersections serving site-generated traffic for intersection capacity analysis.

All site access driveways
Nearest County intersection in each direction from site access driveway(s)

3. Collect intersection turning movement counts for each study intersection during target peak hour(s).

Calculate peak hour factor from analysis of turning movement counts.  Determine heavy truck percentage for intersection capacity analysis.

4. Perform intersection capacity analysis for study intersections under the following conditions:

Existing Year Condition – analysis of (1) existing traffic and (2) existing intersection configuration and control.
Horizon Year No-Build Condition – analysis of (1) existing traffic forecast to horizon year, (2) new traffic generated by other known planned developments, and (3) existing intersection configuration and control with modifications per County or other known planned improvements.
Horizon Year Build Condition – analysis of (1) existing traffic forecast to horizon year, (2) new traffic generated by other known planned developments, (3) traffic generated by subject development, and (4) existing intersection configuration and control with modifications per County or other known planned improvements.
Horizon Year represents the year that the subject development opens under full buildout and operation.
Use intersection capacity analysis software based on Highway Capacity Manual, latest edition.

5. Determine roadway modifications from intersection capacity analysis findings and, as pertinent, Manual on Uniform Traffic Control Devices, traffic signal warrants.