CHAPTER 5

Summary of Findings

The objective of this synthesis was to document state DOT practices regarding the use and adaptations of the HCM LOS framework for traffic capacity and multimodal analysis purposes, including how LOS is defined, whether it is used as a target or standard, and the extent to which big data and advanced technologies affect the LOS analysis and dissemination. This synthesis consists of a literature review, a survey of state DOTs, and case examples of six selected DOTs. This chapter summarizes the synthesis findings and presents recommendations for future research.

Summary of Key Findings

State DOTs use the HCM LOS framework across various stages of transportation projects, including planning, preliminary engineering, and design, as well as operations. Each state has tailored the HCM LOS methodologies to meet specific needs and contexts. Some common implementations of the LOS framework include planning and design of facilities, intersection control evaluation (ICE), interchange access change requests (IACRs), and traffic impact studies. DOTs may use other performance measures in addition to LOS.

Results of the survey and case example interviews indicate that many states use the HCM LOS and related methodologies described in the HCM; however, some DOTs have developed modifications to the HCM methodologies and even to the adopted LOS thresholds. Some DOTs have developed guidance documents that describe their LOS analysis processes. Modifications related to the auto mode include state-specific calibration factors, different LOS thresholds and capacity values for certain facility types (e.g., freeway segments, urban streets, roundabouts, ramp terminals, and alternative intersections), different PCE values, and guidance on the use of saturation flow rate and right-turn-on-red (RTOR). Some DOTs have also modified the multimodal LOS (MMLOS) threshold values.

Although all responding DOTs use an HCM-based method for auto-centric LOS studies, approximately half of the DOTs surveyed (15–20 DOTs, based on facility type) do not use any specific manual for multimodal evaluations. Nine DOTs reported that complexity, inaccuracy, and inability to capture user perception are major reasons for not using the HCM MMLOS framework. One DOT argued that safety-related performance measures are more appropriate for MMLOS. Alternative methodologies include level of traffic stress (LTS), various indices from the NCHRP Research Report 948, simplifications to the HCM multimodal LOS method, and design-related manuals such as the FHWA Bikeway Selection Guide.

State DOTs identified some gaps and limitations regarding the auto mode LOS framework that pertain to the manipulability of LOS, lack of subcategories for LOS F, and inaccuracies in specific methods like freeway weaving and passenger car equivalent (PCE) values on steep roadways. The primary factor for selecting different LOS threshold values is to maintain consistency across similar segment types.

A few DOTs are not considering LOS as a deciding factor or are moving away from it. For example, Oregon DOT uses volume to capacity (v/c) ratio or demand-to-capacity (d/c) ratio as the primary MOE for several applications, despite reporting LOS as part of their analysis.

Twenty state DOTs reported going beyond LOS F for congested conditions and capturing additional performance measures, such as the spatial extent of congestion, the temporal duration of congestion, the amount of excess d/c ratio, the unmet demand, and the change in travel time and speed. Minnesota DOT developed five sub-levels of LOS F based on density values for freeway segments. Florida DOT has also recommended three levels of congestion (lightly, moderately, and heavily congested) based on the average speed along freeway segments, but these congestion levels are not mapped to LOS F subcategories.

Regarding the use of traffic analysis tools for LOS analysis, state DOTs are generally consistent with FHWA’s guidance (FHWA, 2004b). Between 30–37 of the responding DOTs (depending on the facility type) use analytical HCM-based tools and 20–27 of the responding DOTs (depending on the facility type) use microsimulation. However, when using microsimulation, all responding DOTs (except Utah DOT) do not report LOS values, since the microsimulation-based service measures are measured differently than the HCM. Microsimulation and analytical tools are also widely used for visualization purposes and communicating LOS to stakeholders and the public. Depending on the methodology used, some DOTs also use probe data for visualization purposes.

DOTs use emerging technologies and big data to calibrate LOS methodologies, supplement data requirements, and provide real-time congestion monitoring. Some DOTs use big data to avoid analysis steps and directly collect performance measures, while others suggest that these types of data do not impact the LOS criteria or threshold values.

A total of 17 states expressed interest in participating in the case examples interviews: six states were selected. The selection criteria for the six DOTs included the variability in the use of the HCM LOS framework, geographic diversity among the selected states, the level of detail they provided in the survey, as well as their willingness to participate.

Information Gaps and Suggestions for Future Research

The results of this synthesis reveal some common grounds but also differences in the use and adoption of the HCM LOS framework among the state DOTs. Future research could investigate the following:

• Development of LOS F subcategories could assist state DOTs in quantifying congestion and aligning the LOS framework with performance measures representative of congestion, such as v/c ratio, queue lengths, etc.
• Case example DOTs indicated a desire for peer-learning and exchange of knowledge between the DOTs on simplifying and expediting their traffic analysis, applying the newer HCM methods, improving their analysis processes, and managing quality control.
• How emerging technologies and big data can be leveraged to assist in LOS determination and develop a guide that documents these processes that could benefit the DOTs.
• How deficiencies and inconsistencies in the HCM methods and LOS criteria were highlighted through the survey and case examples. It may be beneficial to revisit the HCM LOS thresholds, either working to standardize them or providing a rationale for the existing differences.