Clear, accessible definitions for common urban planning terms.
What Is LOS?
Level of Service (LOS) defines how well vehicle traffic flows along a street or road. LOS is one of the most influential metrics in planning, with critical relevance for both land use and transportation planning.
Level of Service, commonly referred to with the abbreviation LOS, is a metric used to evaluate the performance of roadways and transportation elements, such as intersections, freeway entrances, and transit service, among others. Though LOS provides a qualitative assessment (i.e., a measurement of the quality of roadway performance), LOS requires calculations that rely on quantitative metrics, such as traffic speed, volume, and density.
LOS is used during planning processes to evaluate the need for, and impacts of, project development. Over time, multiple versions of the LOS calculation have been created to evaluate the performance of transportation elements as varied as intersections, pedestrian infrastructure, transit service, and freeway entrances, to name just a few examples. Transportation engineers rely on LOS to determine the size and scope of development projects—i.e., what changes should be implemented and where.
The Specifics of Level of Service
The HCM famously relies on a letter grade system for rating Level of Service (LOS):
|Level of Service
|General Operating Conditions
|Free flow, with low volumes and high speeds.
|Reasonably free flow, but speeds beginning to be restricted by traffic conditions.
|Stable flow, but most drivers are restricted in the freedom to select their own speeds.
|Approaching unstable flow; drivers have little freedom to select their own speeds.
|Unstable flow; may be short stoppages.
|Forced or breakdown flow; unacceptable congestion; stop-and-go.
A blog post by Jean Paul-Rodrigue describes how calculations were made to produce these ratings in the 1994 edition of the HCM.
While some definitions of LOS floating around on the Internet imply that the metric is only applied at intersections, the HCM applies LOS for numerous elements of the transportation system, including, but not limited to:
- Two-lane roadways
- Multilane roadways (4 or more lanes)
- Open freeway segments
- Freeway entrances (merges), exits (diverges), and weaving lanes
- Bicycle facilities
- Pedestrian facilities
LOS was first introduced in the 1965 edition of the Highway Capacity Manual (HCM), published by the Transportation Research Board. The most recent version of the HCM, published in January 2022, included a new pedestrian LOS, among other new and ongoing changes to how LOS is calculated for specific transportation elements. The HCM costs $250 to download, which contributes to the lack of transparency around LOS and its importance to congestion mitigation and land use planning. Some of the other transportation engineering and planning guides that rely on the HCM definition of LOS, such as A Policy on Geometric Design of Highways and Streets, also come with a hefty price tag.
Land Use Applications
Level of Service (LOS) is often used as a critical metric in determining entitlements—or the legal agreements between a government and a landowner to allow proposed developments. Some land use regulations around the country strictly prohibit development of when they would impact the LOS for surrounding streets and intersections. Moreover, environmental impact is frequently measured by LOS, and development approval frequently depends on low or acceptable impacts on LOS for surrounding streets and intersections.Thus, in addition to determining where and how transportation infrastructure investments are made, LOS is also used to prevent land use development projects (e.g., residential, commercial, and industrial uses) along streets and roads.
Because of its central role in so many zoning codes or land use regulations, LOS has been a key factor in enabling the dominance of the automobile in the past century-plus of transportation planning practice in the United States and in other parts of the world, such as Australia, Great Britain, and Northern Ireland. Where roadway performance fails an LOS assessment, new roadway capacity is usually proposed as a congestion mitigation project. Where new residential or commercial developments might worsen LOS for nearby roads and intersections, development proposals fail or they are required to spend money to mitigate LOS effects. As a result, roads and developments tend to be approved farther and farther into the periphery of metropolitan areas, while infill development projects are consistently rejected due to concerns about traffic.
In the car-centric planning politics of the 20th and 21st centuries, LOS is a feature, rather than a bug, resulting in more roads and sprawling development. Numerous studies have suggested that dense urban development with a mix of uses is necessary to reduce automobile trips and vehicles and Vehicle Miles Traveled. A dependence on LOS ensures, however, that infrastructure investments and development patterns will prioritize automobile travel above other concerns, locking the vast majority of the country into a cycle of automobile dependency.
Calls for Reforms
It's become much more common in recent years to see criticism of the overreliance on Level of Service (LOS) in planning processes.
"Decision making based on these grades takes a myopic view of mobility, where the uninterrupted flow of individual vehicles takes precedence over other factors that could improve an area, like efforts to boost accessibility, safety and sustainability," wrote Patrick Sisson for City Monitor, for example, in July 2021.
In 2015, Bill Lindeke wrote that relying on LOS to assess traffic flow is further tipped toward car-centric practices by calculating LOS at peak rush hour—a relatively short window of time during the course of the day. "Rush hour is a cruel tyrant. It grips our lives with grim and gritted teeth," wrote Lindeke in that article.
Writing in October 2013, Angie Schmitt added sprawl and traffic safety to the list of complaints: "Because Level of Service only rewards the movement of motor vehicles, it promotes dangerous, high-speed streets and sprawling land use." As reported by Schmitt in the same article, Pasadena, California has experimented with new metrics for roadway performance, including vehicle speed (where in some cases slower vehicle speeds would be more desirable for traffic safety) and accessibility—both intended to respond to the needs of other users besides the automobile driver.
Other jurisdictions outside of Pasadena have also experimented with LOS reforms. In the most famous example, the California State Legislature in 2013 achieved a benchmark reform of the California Environmental Quality Act (CEQA) law by approving SB 734 (the law was fully implemented in 2020). SB 734 required all environmental studies for proposed projects in the state to switch from LOS to Vehicle Miles Traveled (VMT) as the critical measure of a project's impact. Previously, the state, its local municipalities, and its regional governments had been basing an assessment of a project's environmental consequences based solely on whether the project would create congestion. By focusing on VMT instead of LOS, CEQA now puts the planning onus on the reduction of car trips.
The city of Seattle also considered and eventually adopted LOS reforms in 2019—though these reforms stopped short of abandoning the metric entirely. Because California so recently shifted away from LOS to VMT, the results of the experiment are still unknown. Quantifying and analyzing the effects of these shifts away from a car-centric calculation of LOS will be a long-term question of fundamental relevance to the practice of both land use and transportation planning.