Recurring Traffic Bottlenecks: A Primer
Focus on Low-Cost Operational Improvements (Fourth Edition)
Long Description Figure 1
Figure 1 is a chart of common locations for localized bottlenecks. The chart lists nine common locations and gives a brief description of each location, as follows. (1) Lane Drops: Bottlenecks can occur at lane drops, particularly midsegment where one or more traffic lanes ends or at a low-volume exit ramp. Lane drops might occur at jurisdictional boundaries, just outside the metropolitan area, or at the project limits of the last megaproject. Ideally, lane drops should be located at exit ramps where there is a sufficient volume of exiting traffic. (2) Weaving Areas: Bottlenecks can occur at weaving areas, where traffic must merge across one or more lanes to access entry or exit ramps or to enter the freeway main lanes. Bottleneck conditions are exacerbated by complex or insufficient weaving design and distance. (3) Freeway On-Ramps: Bottlenecks can occur at freeway on-ramps, where traffic from local streets or frontage roads merges onto a freeway. Bottleneck conditions are worsened on freeway on-ramps without auxiliary lanes or short acceleration ramps, where there are multiple on-ramps in close proximity, and when peak volumes are high or large platoons of vehicles enter at the same time. (4) Freeway Exit Ramps: Freeway exit ramps, which are diverging areas where traffic leaves a freeway, can cause localized congestion. Bottlenecks are exacerbated on freeway exit ramps that have a short ramp length, traffic signal deficiencies at the ramp terminal intersection, or other conditions (e.g., insufficient storage length) that may cause ramp queues to back up onto freeway main lanes. Bottlenecks could also occur when a freeway exit ramp shares an auxiliary lane with an upstream on-ramp, particularly when there are large volumes of entering and exiting traffic. (5) Freeway-to-Freeway Interchanges: Bottlenecks occur at freeway-to-freeway interchanges, which are special cases of on-ramps where flow from one freeway is directed to another. These are typically the most severe form of physical bottlenecks because of the high traffic volumes involved. (6) Changes in Highway Alignment: Changes in highway alignment, which occur at sharp curves and hills and cause drivers to slow down either because of safety concerns or because their vehicles cannot maintain speed on upgrades, can cause localized, recurring bottlenecks. Another example of this type of bottleneck is in work zones where lanes may be shifted or narrowed during construction. (7) Tunnels/Underpasses: Bottlenecks can occur at low-clearance structures, such as tunnels and underpasses. Drivers slow to use extra caution, or to use overload bypass routes. Even sufficiently tall clearances could cause bottlenecks if an optical illusion causes a structure to appear lower than it really is, causing drivers to slow down. (8) Narrow Lanes/Lack of Shoulders: Bottlenecks can be caused by either narrow lanes or a lack of roadway shoulders. This is particularly true in locations with high volumes of oversize vehicles and large trucks. (9) Traffic Control Devices: Bottlenecks can be caused by traffic control devices that are necessary to manage overall system operations. Traffic signals, freeway ramp meters, and tollbooths can all contribute to disruptions in traffic flow.