Improved Analysis of Two-Lane Highway Capacity and Operational Performance (2018)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report 91 4. Summary and Recommendations 4.1. Summary 4.1.1. Field Data As the budget for field data collection in this project was limited, the research team sought the assistance of transportation agencies for providing field data. Individuals with the departments of transportation of Oregon, North Carolina, Idaho, Montana, and California assisted the research team with collecting and providing field data. The Oregon sites contained passing lanes, whereas the remainder of the sites did not contain any passing lanes. 4.1.2. Analysis Methodology This project sought to address gaps and limitations in the current two-lane highway analysis methodology for the Highway Capacity Manual. Specific areas investigated included: • Speed-flow relationship • Service measures • Identifying follower status • Accounting for heavy vehicle impacts • Estimating base-free flow speed (BFFS) • Performance of passing lanes • Capacity • Facility scope • Ease of use A brief summary of the accomplishments/findings in each of these areas follows. Speed–Flow Relationship A more realistic speed-flow relationship, versus the current linear form, was developed. This new relationship is non-linear in form and consistent with findings from other countries. The specified mathematical function for this relationship works for both passing lane and non-passing lane segment types (one has a concave shape and the other a convex shape). Service Measures A single, new, service measure was introduced—follower density. Follower density is density multiplied by the percentage of vehicles in a following status. Because of the unique characteristics of two-lane highways and the wide range of platooning characteristics that can result for any given level of traffic demand, follower density more accurately captures the quality of operating conditions than density alone. Follower density has also gained appeal in some other countries (e.g., South Africa, Spain, Brazil, Japan). Identifying Follower Status The current HCM methodology did not explicitly use ‘percent followers’ in the methodology, but the determination of a follower status is implicit in its ‘percent time spent following’ (PTSF)

NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report 92 service measure. The HCM stated that a 3.0-s headway criterion could be used to approximate the percentage of followers for the purpose of approximating PTSF. The analysis methodology developed in this project also uses the criterion of a headway threshold for determining follower status. However, the threshold value was reduced from the current value of 3.0 seconds to 2.5 seconds. This new value was found to better identify following status when the combination of headway and speed were considered for trailing vehicles. While other methods for identifying follower status are potentially more accurate (e.g., Catbagan and Nakamura, 2010), it was felt that these methods were too complex for the intended level of complexity of HCM analysis methods. Accounting for Heavy Vehicle Impacts The current HCM methodology uses the concept of passenger car equivalents (PCEs) to adjust a traffic stream with some percentage of heavy vehicles to an “equivalent” one of passenger cars only. The analysis methodology developed in this project has abandoned the PCE concept and uses the percentage of heavy vehicles directly in the models for estimating the follower density service measure. This approach is simpler, more intuitive, and results in more accurate results when for segments of highway with moderate to steep grades. The new methodology also accounts for the impact of horizontal curvature on heavy vehicles speeds, which is ignored in the current HCM methodology. Estimating Base Free-Flow Speed (BFFS) Specific quantitative guidance for the estimation of BFFS is not provided in the current HCM methodology. It is suggested that the posted speed limit be considered in the estimate of BFFS. In this project, a specific quantitative adjustment based on posted speed limit is included for the estimation of BFFS. Performance of Passing Lanes This project revisited the topic of the effective length of passing lanes (i.e., the distance downstream of the passing lane before traffic stream performance returns to the same level prior to the passing lane). A new function for estimating this value was developed. The topic of the optimum length of passing lanes was also addressed. The optimum length is identified as essentially the length at which any additional length of passing lane will bring minimal additional improvement to the performance measures. The values identified from this project are fairly consistent with those provided previously in the HCM. This project also investigated the ‘2+1’ design that has become popular in Europe. Guidance on expected traffic performance improvement with this type of configuration was developed. Capacity Accurate identification of capacity for two-lane highways has always been elusive. This is because operational performance on two-lane highways often becomes intolerable at volume-to-capacity ratios well below 1.0; in which case these two-lane highways get converted to multilane highways. The field data obtained and analyzed as part of this project did not offer enough conclusive evidence to justify revising the capacity value recommended in the current HCM methodology—

NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report 93 1700 pc/h/ln. However, of the field data collected for this project, two locations were found to experience high flow rates, with maximum flow rates similar to the 1700 pc/h/ln value. Facility Scope The current HCM methodology is designed to provide a segment-level analysis. The analysis methodology developed in this project provides a method for combining the analysis of multiple contiguous segments into a facility-level analysis. Ease of Use The analysis methodology resulting from this project has introduced several features that improve the ease of use of the methodology relative to the current one in the HCM: • Elimination of tables that require interpolation • Treating trucks explicitly, rather than through passenger car equivalent values • A single service measure • Elimination of the PTSF measure, which was difficult, if not impossible, to measure in the field. 4.1.3. Simulation This project has identified two modern simulation tools that are capable of effectively and accurately modeling two-lane highways: SwashSim and TransModeler. The former simulation tool was used to generate the data for model development, while the latter tool was used to test the results of the former tool for a sampling of the experimental design scenarios. 4.2. Recommendations It is recommended that the proposed methodology described in Appendix G replace the current HCM methodology (i.e., Chapter 15 of the HCM 6th edition). While the results from this project have addressed most of the gaps and limitations of the current HCM methodology, there are several areas that could still benefit from further research, such as: • Testing the relationship between free-flow speed and posted speed limit across a wider range of posted speed limits. The large majority of field sites studied in this project had a posted speed limit of 55 mi/h. • Identify other high flow rate sites in order to further assess current estimates of capacity. • Conduct research on driver perception of operating conditions and revise the follower density LOS thresholds as necessary. • Build upon the work by Li and Washburn (2014) to further validate and refine a methodology for analyzing two-lane highways with occasional signalized intersections.