When is a road congested?

downloadThere are many factors that contribute to a road feeling congested. The biggest factors are how many vehicles are using the road (demand) and how many lanes are provided on the road (supply). Transportation professionals describe this demand and supply in terms of volume to capacity ratios. A road with a volume to capacity ratio of almost 1.0 (demand equal to supply) will definitely feel congested to motorists.

Here are factors related to the volume (demand) side of the ratio:

  • The number of vehicles using the road, both during a peak hour and during the course of the day.
  • The number of pedestrians and bicyclists (non-motorized users), also during a peak hour and over the course of a day.
  • Vehicle speeds.
  • The directional distribution of traffic (a heavy commuter route may have 70% of the traffic on that road in the morning rush hour heading into the job center, while a balanced road in the core urban area may have a 50/50 split).
  • The mix of heavy vehicles in the traffic stream. Semi-trucks take up much more of the capacity than passenger vehicles.

Here are factors related to the capacity (supply) side of the ratio:

  • The number of lanes on the road.
  • The number of traffic signals, stop signs, interchanges or roundabouts per mile of road.
  • The number of access points per mile of road (driveways and cross streets).
  • The number and type of transit use (buses, light rail, taxis, etc.).
  • Availability of on-street parking and type of parking (parallel, 90-degree, etc.).
  • The terrain (twisty two lane roads in the mountains have a different capacity than straight two lane roads on the flat prairie).
  • The presence of, or lack of, bicycle lanes, trails, and sidewalks.
  • The posted speed limit.

Junction points (interchanges on freeways and intersections on other roads) are typically the sticking points that traffic engineers analyze to develop a detailed microscopic view of how a roadway corridor operates. This level of detail is typically needed when developing transportation improvement projects or finalizing design plans. However, transportation planners have developed rules of thumb based on a host of assumptions that provide a quick not congested/almost congested/congested macroscopic categorization of roads based on the average daily traffic volumes, or ADTs, on the road.

Below is a rough guide for how much traffic can be accommodated on different types of roads. If the daily volume is below the threshold, the road is not congested. If the daily volume is in the range, the road is almost congested. And if it is over the range the road is definitely congested.

  • 2-Lane (one in each direction with left turn lanes at busy intersections and coordinated signals), undivided – 11,500 to 19,900 vehicles per day[1]. It should be noted this is the physical capacity. Research from UC Berkley indicated quality of life along a residential street is negatively impacted when the daily volumes are greater than 1,000 vehicles per day. Also, the maintenance costs related to maintaining gravel roads makes paving them cost effective when the road carries more than 300 to 400 vehicles per day.
  • 4-Lane, undivided (two in each direction with left turn lanes at busy intersections and coordinated signals), – 23,500 to 37,900 vehicles per day1.
  • 6-Lane, divided (three in each direction with left turn lanes at busy intersections and coordinated signals), – 34,800 to 54,300 vehicles per day1.
  • 4-Lane Freeway (2-lanes in each direction with a median) – 34,300 to 75,300 vehicles per day[2].
  • 6-Lane Freeway (3-lanes in each direction with a median) – 51,400 to 112,900 vehicles per day2.
  • 8-Lane Freeway (4-lanes in each direction with a median) – 68,600 to 150,500 vehicles per day2.

Although we still use volume to capacity ratios in our planning because they provide a simple metric, transportation planners are starting to focus on the reliability of the commute. A consistent twenty minute commute is better than a commute that usually takes twenty minutes but randomly takes forty minutes a quarter of the time. Measuring commute reliability is a lot more difficult than measuring volume to capacity ratios, but companies like INRIX and AirSage are gathering the big data necessary to make the reliability calculations.

[1] HCM 2010 Highway Capacity Manual, Transportation Research Board of the National Academies, Exhibit 16-14

[2] HCM 2010 Highway Capacity Manual, Transportation Research Board of the National Academies, Exhibit 10-9

Photo Credit – StrongTowns.org

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