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3.0 Highway Truck Bottleneck Typology

A typology of truck bottlenecks was developed to categorize bottlenecks clearly and consistently. A typology is necessary to avoid double counting when calculating truck hours of delay and to establish—for future for policy and program analysis work—a framework for attaching strategies and costs for congestion mitigation to each type of bottleneck.

To develop the typology we reviewed relevant literature and information about highway bottlenecks from several sources:

For the purposes of this paper, we recommend that highways bottlenecks for trucks be defined by a combination of three features: the type of constraint, the type of roadway, and the type of freight route. The elements for describing bottlenecks are summarized in Table 3.1.

Table 3.1 Truck Bottleneck Typology
Constraint Type Roadway Type Freight Route Type
Lane-DropFreewayIntercity Truck Corridor
InterchangeArterialUrban Truck Corridor
Intersection/SignalCollectors/Local RoadsIntermodal Connector
Roadway Geometry Truck Access Route
Rail Grade Crossing  
Regulatory Barrier  

More detailed definitions of each element are provided below, but as an example, a truck bottleneck may be caused by a lane drop that creates insufficient lane capacity on a freeway used as an intercity truck corridor, or a bottleneck may be caused by lane drop on an arterial that serves as a urban truck corridor. Similarly, a truck bottleneck may be caused by congestion at an interchange on a freeway serving as an intercity truck corridor, or a truck bottleneck may be caused by poorly timed traffic signals at intersections on an arterial road that serves as an urban truck corridor.

Several combinations are not used; for example, neither signalized intersections nor rail grade crossings exist on freeways; and most truck access routes are by definition on arterial roadways or collectors/local roadways, not freeways. Other combinations such as an interchange involving a collector/local road are rare.

Finally, while the paper identified and examines a few bottlenecks on collector/local roads serving as intermodal connectors and truck access routes, because of data limitations, the majority of attention in the paper is focused on bottlenecks that occur on freeways and arterials.

The six capacity constraints are:

  1. Lane-Drop Constraint. An example of this type of bottleneck would be a lane drop, where a highway narrows from three to two lanes or two lanes to one lane, reducing throughput and creating traffic queues. These bottlenecks typically affect one direction of traffic flow.
  2. Interchange Constraint. An example of this type of bottleneck would be an urban interchange connecting two Interstate highways (or an interchange connecting an Interstate highway and a major arterial) where the geometry of the interchange, traffic weaving and merging movements, and high volumes of traffic reduce throughput and create traffic queues on the ramps and the mainlines. Severely congested interchanges may cause queues on one or both highways. Where interchanges are closely spaced, queues from one interchange may create additional bottlenecks at upstream interchanges, producing a series of closely linked bottlenecks.
  3. Intersection/Signal Constraint. An example of this type of bottleneck would be an urban or suburban arterial road with closely spaced intersections operating at or near capacity, often with poorly timed signals. As with queues at closely spaced interchanges, queues at one congested intersection often impact traffic flow at other intersections upstream of the affected location. These bottlenecks may affect flows in both directions on all intersecting roadways.
  4. Roadway Geometry Constraint. An example of this type of bottleneck would be a steep hill, where heavily loaded trucks must slow to climb and descend. The total volume of traffic, the number of heavy trucks, the number of lanes, and the presence or absence of an additional climbing lane determine the throughput of these bottlenecks. Other roadway geometry barriers include curves with insufficient turning radii for trucks (usually on two-lane roadways), bridges with gross vehicle weight limits that force trucks to make long detours, and tunnels with reduced overhead or side clearance.
  5. Rail Grade Crossing Constraint. An example of this type of bottleneck would be a highway-rail at-grade crossing where an urban roadway carrying high volumes of truck traffic crosses a rail line carrying high volumes of passenger or freight trains. Frequent gate closings may cause long traffic queues in both directions on the roadway.
  6. Regulatory Barrier Constraint. Examples of this type of bottleneck include toll barriers, international border custom inspection stations, and increasingly, security inspection checkpoints. Also included in this category are permanent safety, hazardous materials (hazmat), and weight restrictions that prohibit truck movements across a bridge, through a tunnel, or along a road, forcing trucks to make long detours.

The three roadway types are:

  1. Freeways. This group includes Interstates, expressways, toll roads, major state highways, and other limited-access (typically divided) highways with multiple lanes and access control.
  2. Arterials. This group includes major state and city roads. They are typically multilane, but not divided roadways. In urban areas, they carry much of the traffic circulating within the urban area.
  3. Collectors/Local Roads. Collectors are typically two-lane roads that collect and distribute traffic to and from the freeway and arterial systems, proving connections to and among residential neighborhoods and commercial and industrial areas.

The four types of freight routes are:

  1. Intercity Truck Corridors. Intercity truck corridors are transcontinental and interregional routes, using rural Interstate highways and rural state highways. Almost all these corridors are designated as truck corridors on the National Truck Network and state truck networks.
  2. Urban Truck Corridors. Urban truck corridors are Interstate highways and major state and city arterials that serve both local distribution and through moves. Most but not all of these corridors are designated as truck corridors on the National Truck Network, and state and city truck networks.
  3. Intermodal Connectors. Intermodal connectors are the "last mile" of National Highway System roadway connecting major port, airport, rail, or truck terminals to intercity routes.
  4. Truck Access Routes. Truck access routes include designated truck routes to industrial or commercial zones, warehousing and distribution centers, central business districts, and suburban centers. The category includes local, urban, and rural routes not designated as urban truck corridors or intermodal connectors.

The typology is not exhaustive. The categories have been designed so that they can be broadened when additional detail is needed for future studies. For example, roadway capacity constraints could be expanded to include temporary operational constraints such as roadway construction work zones and emergency closures for crashes and other incidents. These are not addressed in this white paper because comprehensive, nationwide data on these capacity constraints is not readily available. Similarly, the category "freeways" could be subdivided into its component roadways—Interstates, expressways, toll roads, major state highways, other limited-access highways—and engineering cost estimates assigned to each.

In urban areas, the categories also could be described by their role in an urban system. For example, freeway/urban truck corridors could be further defined as circumferential urban Interstate highways or as radial arterial roadways used as urban truck corridors, etc. Greater definition would require detailed examination of each bottleneck in the context of a metropolitan map.

More definition also could be provided for the type of freight route. This was not done for the initial typology because data were not readily available to clearly differentiate freight route functions. A capacity bottleneck on a rural Interstate highway can be readily classified as impacting an intercity truck corridor. However, a capacity bottleneck on an urban Interstate highway such as a circumferential beltway may affect transcontinental truck trips, intraregional truck trips, metropolitan distribution trips, and local pickup and delivery operations. Most urban truck corridors serve two or more of these functions. Information on the length of the truck trips passing through the bottleneck may indicate whether the majority of trips are longer or shorter, but in most cases does not clearly differentiate the type of freight route.

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