|This fact sheet is an archived publication and may contain dated technical, contact, and link information|
|Publication Number: FHWA-HRT-09-058
Date: October 2009
Quadrant Roadway Intersection
FHWA Contact: Joe Bared,
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This document is a technical summary of the Federal Highway Administration report, Alternative Intersections/Interchanges: Information Report (AIIR) (FHWA-HRT-09-060).
Today's transportation professionals, with limited resources available to them, are challenged to meet the mobility needs of an increasing population. At many highway junctions, congestion continues to worsen, and drivers, pedestrians, and bicyclists experience increasing delays and heightened exposure to risk. Today's traffic volumes and travel demands often lead to safety problems that are too complex for conventional intersection designs to properly handle. Consequently, more engineers are considering various innovative treatments as they seek solutions to these complex problems
The corresponding report, Alternative Intersections/Interchanges: Informational Report (AIIR) (FHWA-HRT-09-060), covers four intersection designs and two interchange designs that offer substantial advantages over conventional at–grade intersections and grade separated diamond interchanges. The AIIR provides information on each alternative treatment covering salient geometric design features, operational and safety issues, access management, costs, construction sequencing, environmental benefits, and applicability. This TechBrief summarizes information on one of these alternative intersection designs–the quadrant roadway (QR) intersection (see figure 1).
A QR intersection is a promising design for an intersection of two busy suburban or urban roadways. The intersection works by rerouting all four left–turn movements at a four–legged intersection onto a road that connects the two intersecting roads. Figure 1 shows the geometry of a QR intersection where the connection road is placed in the southwest quadrant. The location of the connector road depends on traffic flow and availability of right–of–way. Figure 2 shows how all four of the left–turning movements are rerouted over the connector road. This design prohibits all left turns at the main intersection and therefore allows a simple two–phase signal to process the remaining through and right–turn movements. Both junctions of the connector road are typically signalized.
The QR intersection concept was first published in 2000.(1,2) Since then, its design has been explored by others, including the Federal Highway Administration (FHWA). However, as of July 2009, no known highway agency has implemented a QR intersection in the United States.
The primary design considerations of the QR intersection are as follows:
Traffic Signal Control
A QR intersection needs three sets of signal controlled junctions–the main intersection with two signal phases and two secondary intersections at the ends of the connecting road with three signal phases each. These junctions are shown in figure 3.
The traffic simulation software VISSIM was used to compare the operational performance of QR intersections to conventional intersections. Four intersection geometric design cases of QR and conventional intersections were simulated under four traffic volumes scenarios. The findings for throughput showed a 5- to 15-percent increase for travel time and a 5– to 20–percent reduction compared to conventional intersections.
Since a complete QR intersection has not yet been built, safety data are not available. Nonetheless, the QR has 28 conflict points more widely spread out than compared to 32 at a conventional intersection. This reduction in conflict points suggests the QR may be safer, although a definitive relationship between conflicts and safety has not yet been established.
QR intersections will typically be spot treatments. They are most applicable where the following exists:
The QR intersection increases operational efficiency through a congested intersection by moving the left turns away from the main intersection and allowing a two–phase signal at the main intersection. While additional right–of way would be needed in the connection road quadrant, and extra cost would be necessary to build the connecting roadway, the QR intersection could be used to reduce congestion at a busy intersection in a developing area. It could also serve as a temporary congestion reliever until a grade–separated solution can be built. In addition, the QR intersection accommodates pedestrians well. More details can be found in the full AIIR available from the FHWA.
Topics: research, safety, intersection safety, interchanges, Alternative Intersection Design, quadrant roadway intersection, access management
Keywords: research, safety, Quadrant roadway intersection, QRI, QR, Alternative intersection