Estimating HCM Default Parameters for Louisiana [Summary]

Rahman, M. Ashifur · 2025 · ROSA P / Louisiana Transportation Research Center

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Summary

This study addresses the need for region-specific intersection capacity parameters in Louisiana, motivated by evidence that national default values from the Highway Capacity Manual (HCM) may not accurately reflect local traffic conditions. Accurate estimation of intersection capacity is critical for effective roadway planning, design, and Transportation Systems Management and Operations (TSMO) initiatives. The research specifically aimed to estimate the saturation flow rate (SFR) for signalized intersections and analyze critical headway (CH) and follow-up headway (FH) for stop-controlled intersections to improve the precision of traffic analysis in the state. The methodology involved collecting statewide intersection traffic flow data from 511 video sources and intersections selected by Louisiana Department of Transportation and Development (DOTD) District Engineers. After filtering for adequate sample sizes, the study analyzed 51 signalized and 26 two-way stop-controlled (TWSC) four-leg intersections across eight Louisiana districts. Intersection geometry was categorized using Google Earth Street View and DOTD pavement management data. For signalized intersections, the HCM Saturation Headway Method was applied to determine local base saturation flow rates (BSFR). For TWSC intersections, CH was estimated using Maximum Likelihood Estimation, while FH was determined via Direct Measurement. Additionally, LASSO regression was utilized to identify factors influencing SFR and CH. The results indicated that Louisiana’s local BSFR for signalized intersections averaged 1,655 pc/h/ln, which is significantly lower than the HCM default of 1,900 pc/h/ln. District-level SFRs ranged from 1,537 pc/h/ln in Lafayette to 1,773 pc/h/ln in Baton Rouge. Regression analysis revealed that SFR increased with lane width, number of lanes, exclusive turning lanes, approach grade, speed limit, and peak-hour conditions, but decreased with the presence of heavy vehicles and during off-peak periods. For TWSC intersections, observed CH values (7.1–10.1 seconds) and FH values (5.9–6.5 seconds) exceeded HCM default ranges. CH increased with heavy vehicles and higher speed limits but decreased with higher Annual Average Daily Traffic (AADT), more rejected gaps, and additional major street lanes. The study concludes that using locally estimated parameters reduces uncertainty in traffic analysis outcomes and better captures regional driving behavior, particularly at low-volume minor street approaches. It recommends adopting the statewide BSFR average of 1,655 pc/h/ln as a planning baseline, with adjustments for specific projects using HCM 7th Edition equations. Functional class distinctions were also noted, with Principal Arterials averaging 1,684 pc/h/ln. The authors advise caution when applying average values to individual intersections, recommending context-specific adjustments and validation with local data. Future research is suggested to explore other intersection configurations, such as roundabouts, and to integrate emerging technologies like Vehicle-to-Everything (V2X) systems and Connected and Automated Vehicles (CAV) into capacity methodologies.

Key finding

Louisiana's local base saturation flow rate is significantly lower than the HCM default, and critical and follow-up headways at stop-controlled intersections are higher than national averages, necessitating region-specific parameter adjustments.

Methodology

field_study

Sample size: 77

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

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• traffic density