Defining LOS criteria for congested traffic conditions

Abstract: Defining LOS criteria for congested traffic conditions
The Highway Capacity Manual (HCM) defines the Level of Service (LOS) as a “quantitative stratification of a performance measure or measures representing quality of service.” (HCM7, 2022). For uninterrupted flow facilities such as freeways and multilane highways, density is the selected service measure used to compute LOS. The HCM defines LOS F when the ratio of demand to capacity exceeds 1.0 or when the service measure exceeds a certain value. After LOS F, no further information is given on the severity of different degrees of congestion. This study aims to define LOS criteria for congested conditions, using data from a freeway segment in California. To conduct the analysis, congested events were identified using the HCM breakdown definition (i.e., these occur when speed drops below 25% of the free-flow speed and lasts for at least 15 minutes, resulting in queuing upstream of the bottleneck). All speed-flow data points during these congested events were considered for the analysis. A K-means clustering algorithm was used in this study to effectively categorize LOS conditions for these traffic conditions. This clustering method grouped data into distinct LOS, representing various degrees of traffic congestion ranking from mild to severe congestion. The results provide valuable insights into how traffic conditions vary within congested environments and offer potential applications for transportation planners and engineers to better manage urban traffic flow. Ultimately, this research contributes to more efficient traffic management and policy-making for congested areas.

References:
National Academies of Sciences, Engineering, and Medicine. 2022. Highway Capacity Manual 7th Edition: A Guide for Multimodal Mobility Analysis. Washington, DC: The National Academies Press. https://doi.org/10.17226/26432.