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TRB's National Cooperative Highway Research Program (NCHRP) Web-Only Document 255: Improved Analysis of Two-Lane Highway Capacity and Operational Performance supplements the sixth edition of the Highway Capacity Manual (HCM). Specifically, this project includes the following updates:
- the development of a more realistic speed-flow relationship
- the introduction of a new service measure—follower density
- a new headway threshold value to better identify follower status
- development of a percent-followers flow relationship
- elimination of passenger car equivalent (PCE) values and direct use of percentage of heavy vehicles in the models for performance measure estimation
- the inclusion of a quantitative adjustment based on posted speed limit for the estimation of base free-flow speed (BFFS)
- the development of new functions for passing lanes—effective and optimal lengths and performance measure improvements for 2+1 sections
- the development of a method for combining the analysis of multiple contiguous segments into a facility-level analysis
This project also introduced features to improve the ease of use of the methodology in the HCM, such as the elimination of tables requiring interpolation, treating trucks explicitly instead of through PCE values, using a single service measure and eliminating the PTSF measure.
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