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Reconsidering Intersection Skew Angles, Safety, and Operations

      For both safety and operational reasons, roadway designers hold that it is desirable that two intersecting roadways meet at an angle of 90 degrees. However, because of constraints, some roadways intersect at angles of less than 90 degrees. Such locations are referred to as skewed intersections, and the difference between 90 degrees and the smallest acute angle between the intersection legs is referred to as the intersection skew angle.
      The AASHTO design guides for highways and streets have for a long time stated that the intersection angle can be as small as 60 degrees. Many transportation agencies have accepted this in their geometric design policies. A search of literature identified little research on the subject, but the research that was found cast doubt on the long-accepted minimum value of 60 degrees, and raises the question: Are current design policies on intersection skew angles based on knowledge or on conjecture?
      With the acute angle to the right side of vehicles stopped at an intersection, Gattis and Low (TRR 1612, 1998) analyzed the angle at which drivers’ lines-of-sight were blocked by the right-sides of the vehicles which they were driving. They found that the minimum angle to afford even stopping sight distance varied with the speed on the through roadway, and that minimum angles of 70o or more, depending upon the through-road speed, may be more appropriate. Son et al. (2002) also analyzed the right lateral visibility of passenger and heavy vehicles and their influence at left-skewed intersections, taking the right center pillar or B-pillar as visual limitation. Based on providing intersection sight distance, they concluded that obliquities greater than 25 degrees are excessive for heavy vehicles, and for cars the safety is obtained with a 20 degrees maximum angle. Garcia (2005) in Spain evaluated the effects of available lateral visibility at skewed intersections, and proposed safe skew angles of no less than 70 degrees for crossing maneuvers.
      Hanna et al. (TRR 601, 1976) found that three-leg Y intersections had accident rates approximately 50 percent higher than there-leg T intersections, suggesting an effect of intersection skew angle. McCoy et al. (Nebraska Department of Roads RES1, 1994) found that accidents increase with increasing skew angle at rural two-way stop controlled intersections. Kulmala (1995) in Finland found that acute and obtuse skew angles affected safety differently. Harwood et al. (FHWA-RD-99-207, 1999) selected an accident modification factor (AMF) for intersection skew angle, based on a negative binomial regression model, for application to STOP-controlled intersections on rural two-lane highways in FHWA’s Interactive Highway Safety Design Model (IHSDM). An Australian research by Arndt and Troutbeck (2005) took into account the observation angle, i.e. a measure of the degree that minor road driver at the intersection need to look sideways or backwards in order to view vehicles on the major road. An increase in the observation angle will increase angle-minor accident rates.
None of these research results are considered sufficiently definitive to form a basis for reevaluation of the appropriate geometric design policy for intersection skew angle. The FHWA Highway Design Handbook for Older Drivers and Pedestrians (2001) has recommended that intersection skew angles be reduced for the benefit of older drivers, but the handbook offers no quantitative estimate of the benefit to older drivers, or to motorists in general, from doing so.  An FHWA document (Rodegerdts et al. 2004) states that significant skews produce larger intersection areas, which are not only more difficult for motorists and pedestrians to navigate, but more costly to construct and maintain. The skew angle and larger area can make turns more difficult, increase pedestrian exposure, and reduce capacity. The Australian Guide to Traffic Engineering Practice -- Part 5: Intersections at Grade (Austroads 2005) limits the skew of an intersection, between 70 and 110 degrees. This guideline proposes visibility angles from vehicles, including sight restrictions due to vehicle design. The angles of vision by mirror (25 degrees) are wider than those obtained by Garcia (2005), with 20 degrees and 16 degrees, for the left and right mirror, respectively. At each point where a vehicle has to give way or is about to enter a traffic stream, the vehicle paths, and orientation should be developed with these visibility angles in mind.
      The objective of this research is to examine operational and safety factors, in light of different environments and scenarios, that are related to the angle at which two roadways can intersect, and provide more definitive guidance as to the desirable minimum skew angles in different environments and scenarios.
      The different environments and scenarios considered should include rural and urban intersections, three- and four-leg intersections, STOP- and signal controlled intersections. The research should consider the effect on safety and operations of the magnitude of the intersection skew angle and the orientation of the intersection leg to approaching traffic (e.g. acute vs. obtuse angle). The actual visibility angles from vehicles, including sight restrictions due to vehicle design and diminished capabilities of older drivers, should also be determined.
      The research should focus on intersections with angles between 60 and 75 degrees and should assess what safety or operational benefits would result from increasing the current minimum intersection angle of 60 degrees. The research should also assess the potential for increased construction costs and other impacts if the minimum intersection angle were to be increased. The assessment of the need for changes in intersection skew angle should consider both the costs and the benefits of any proposed change in design policy. If a change in design policy is recommended, draft text for revision of the AASHTO Green Book should be provided in the final report of the research.
Recommended Funding: $400,000
Research Period: 3 years
This research topic was selected by the AASHTO Technical Committee on Geometric Design, the TRB Committee on Geometric Design, and the TRB Committee on Operational Effects of Geometrics at their combined meeting in June 2004 as a priority issue from among a broader set of problems considered. The two TRB committees again targeted this topic during their combined meeting in July 2009.  The research is needed to address an unresolved issue in highway geometric design. The research results can be implemented through incorporation in the AASHTO Green Book.
  1. Arndt, O. and R. Troutbeck. (2005). “Relationship between unsignalised intersection geometry and accident rates.” Proc., 3rd International Symposium on Highway Geometric Design, TRB, Chicago, June 29-July 1.
  2. AASHTO (2001). A policy on geometric design of highways and streets, Washington DC.
  3. AUSTROADS (2005). Guide to traffic engineering practice —Part 5: intersections at grade, Sydney.
  4. Garcia, A. (2005). “Lateral vision angles and skewed intersections design.” Proc., 3rd International Symposium on Highway Geometric Design, TRB, Chicago, June 29-July 1.
  5. Gattis, J. L. and S.T. Low. (1998). “Intersection angle geometry and the driver’s field of view.” Transportation Research Record 1612. TRB, Washington DC.
  6. Hanna, J. T., T. E. Flynn, and L. T. Webb (1976). "Characteristics of intersection accidents in rural municipalities." Transportation Research Record, 601, TRB, Washington DC.
  7. Harwood, D.W., F.M. Council, E. Hauer, W.E. Hughes, and A. Vogt. (1999). Prediction of the expected safety performance of rural two-lane highways, Publication No. FHWA-RD-99-207, U.S. DOT, Federal Highway Administration, Turner-Fairbank Highway Research Center, McLean, VA.
  8. Kulmala, R. (1995). Safety at rural three- and four-arm junctions. Development and application of accident prediction models, VTT Publications.
  9. McCoy, P. T., E. J. Tripi, and J. A. Bonneson (1994). Guidelines for realignment of skewed intersections, Nebraska Department of Roads, Research Project Number RES1 (0099) P471.
  10. Rodegerdts, L.A., B. Nevers, B. Robinson, J. Ringert, P. Koonce, J. Bansen, T. Nguyen, J. McGill, D. Stewart, J. Suggett, T. Neuman, N. Antonucci, K. Hardy, K. Courage (2004) Signalized Intersections: Informational Guide, FHWA-HRT-04-091, Federal Highway Administration, McLean, VA, pp. 45-46.
  11. Son, Y. et al. (2002). “Methodology to calculate sight distance available to drivers at skewed intersections.” Transportation Research Record, 1796. TRB, Washington DC.

Sponsoring Committee:AKD10, Performance Effects on Geometric Design
Date Posted:09/15/2010
Date Modified:09/15/2010
Index Terms:Intersections, Skew angles, Traffic safety, Highway operations, Highway design, Traffic accidents,
Cosponsoring Committees: 
Operations and Traffic Management
Safety and Human Factors

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