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Vehicle Paths on Horizontal Transition Curves

The American Association of State Highway and Transportation Officials” (AASHTO) Policy on Geometric Design of Highways and Streets (Green Book) provides some general guidance on the use of transition curves to connect a straight or nearly straight horizontal alignment and a curve which requires the driver to significantly lower their speed to negotiate the curve. One method is to employ the use of a spiral curve to connect the two alignments, usually of a length greater than the length required for the superelevation transition from one curve to the other. Another method is to use a flatter curve of a similar length with a radius not exceeding a 1.5:1 ratio to the sharper curve’s radius to create a compound curve to assist the driver on open highways. A 2:1 ratio is suggested as the maximum for compound curves on turning roadways. A third option (not directly discussed in text) is to provide no transition but allow the driver to adjust their path based on an assessment of the roadway ahead. No positive guidance is provided in the Green Book as to which method is most suitable or how the methods compare with each other.
The issue is of significance if the driver is forced to adjust their path in a way that the vehicle may shift outside the lane lateral limits and encroach on an approaching lane or onto a shoulder not intended for vehicle traffic. Rural settings may have narrow shoulders that could allow the vehicle to easily depart a paved surface and cause loss of control of the vehicle. Rural and suburban settings may have pedestrian and/or bicycle traffic on the shoulders, which could increase the probability of a crash. On turning roadways such as freeway exit terminals, drivers may encounter loop curves on which the smaller radius requires a substantial reduction (more than 20 mph or 32 km/h) in operating speed. Frequent “rollover” accidents at these locations may be the result of a driver misinterpreting the transition path required to negotiate the curve.
State and local agencies often face a situation where a transition curve would be recommended in new design, but they are dealing with an existing condition in which no transition was provided. Guidance needs to be developed to assist these agencies in providing a “transition” without rebuilding major portions of the roadway. This may be in the form of a spiral which “best fits” a vehicle’s establish path in negotiating the transition.
There has been a significant amount of research conducted in the past related to transition curve design. However, must of the published literature focuses on vehicle operating speeds and transition curve design along two-lane rural highways. Firestine et al. (1989) documented several interchange ramp features that contribute to truck crashes, including poor transitions to superelevation, abrupt changes in compound curves, and short deceleration lane lengths followed by sharp horizontal curves. The AASHTO Strategic Highway Safety Plan has identified a need for improved interchange ramp design to reduce the frequency of truck-related fatalities, underscoring the importance of the proposed research.  The following list of references contains documentation related to the proposed research and the variety of methods used to study vehicle transition curve design.
Bella, F. New Model to Estimate Speed Differential in Tangent-Curve Transition. Advances in Transportation Studies, Vol. 15, July 2008, pp. 27-36.
Bonneson, J. NCHRP Report 439: Superelevation Distribution Methods and Transition Designs. TRB of the National Academies, Washington, DC, 2000.
Council, F. M. Safety Benefits of Spiral Transitions on Horizontal Curves on Two-Lane Rural Roads. Transportation Research Record 1635, TRB, National Research Council. Washington, DC, 1998, pp. 10-17.
Firestine, M., H. McGee, and P. Toeg. Improving Truck Safety at Interchanges: Summary Report. Report No. FHWA-IP-89-024, Federal Highway Administration, Washington, DC, 1989.
Glennon, J. C. and G. D. Weaver. The Relationship of Vehicle Paths to Highway Curve Design. Project No. 2-8-68-134, Texas Transportation Institute, Arlington, TX, 1971.
Passetti, K. A. and D. B. Fambro. Operating Speeds on Curves With and Without Spiral Transitions. Transportation Research Record 1658, TRB, National Research Council. Washington, DC, 1999, pp. 9-16.
Perco, P. Desirable Length of Spiral Curves for Two-Lane Rural Roads. Transportation Research Record, Journal of the Transportation Research Board, No. 1961, TRB of the National Academies, Washington, DC, 2006, pp. 1-8.
Perco, P. Comparison Between Vehicle Paths Along Transition Sections With and Without Spiral Curves. Proceedings from Third International Symposium on Geometric Design, Chicago, IL, 2005.
Zegeer, C. V., et al. Safety Effects of Geometric Improvements on Horizontal Curves. Transportation Research Record 1356, TRB, National Research Council, Washington, DC, 1992, pp. 11-19.
The objective of this research is to (1) analyze and summarize results of previous research, (2) identify specific relationships that need to be studied to complete the knowledge base needed to form a unified policy on transition curves, and (3) formulate the next steps in implementing the research.
This research should be completed in two phases. The first phase should be undertaken to determine the nature of the problem (i.e., are horizontal transition curves causing excessive lateral vehicle shifts?). Specific tasks required to complete this first phase include the following:
  • Review extant literature related to horizontal curve transition design and any research that has been completed related to vehicle paths and operating speeds on horizontal curves, including compound circular curves, spiral transitions, and tangent-to-curve transitions;
  • Develop an experimental and/or field study protocol to collect data to study the nature of vehicle paths and operating speeds on transition curves, focused primarily on freeway exit ramps. If a field study is undertaken, it is important to find data collection sites where transition curves are used and similar sites where they are not. For example, Ohio DOT uses transition curves on freeway exit ramps while Indiana DOT does not. Likewise, Oregon DOT uses transition curves on freeway exit ramps while the California DOT discourages the use of transition curves.
  • Analyze the vehicle path and operating speed data to determine the magnitude of vehicle shifts and speed reductions on turning roadways with and without transition curves. Compare these results to literature that compares lateral vehicle path shifts and operation speed reductions to accident occurrence.
If the results from the first phase of the research suggest that a 1.5:1 or 2:1 compound curve ratio is a significant safety (or safety surrogate) issue on freeway exit terminals, then a second phase of the project should be completed. This second phase would involve the following tasks:
  • Collect crash frequency and severity data along freeway exit terminals with and without compound circular curves (including spiral transitions and simple tangent-to-curve transitions). 
  • Perform appropriate statistical analyses to develop a revised compound circular curve ratio for turning roadways.
  • Develop design guidance based on the findings for incorporation into the AASHTO Policy on Geometric Design of Highways and Streets. If appropriate, separate design guidance should be developed for new construction and reconstruction projects.
As stated above, this particular research would be the first part of a 2-part study to clarify the importance of transition curves, compound or spiral, and to establish guidelines on their implementation in new design and retrofitting existing design. The results would be incorporated into future editions of AASHTO’s policy on design.
Recommended Funding:
It is estimated that $150,000 would be required to complete the first phase of the proposed research. If the second phase of the research is required to develop the appropriate transition curve design guidance, an additional $350,000 would be required.
Research Period:
It is estimated that one year be required to complete the first phase of this research. If necessary, the second phase would require two additional years to complete.
The proposed research was identified by the TRB Committees on Geometric Design and Operational Effects of Geometrics. The urgency and potential payoff of the research is high. The research supports the AASHTO Strategic Highway Safety Plan for reduced truck-related crashes. The findings from the research should be included in the AASHTO Green Book for new and major reconstruction projects. In particular, a revised compound curve design ratio should be addressed in the recommended guidelines for turning roadways.

Sponsoring Committee:AKD10, Performance Effects on Geometric Design
Source Info:TRB Committee on Geometric Design (AFB10)
Date Posted:09/15/2010
Date Modified:09/15/2010
Index Terms:Highway design, Geometric design, Curves (Geometry), Highway curves, Travel patterns, Traffic safety, Data collection, Highway operations, Traffic speed,
Cosponsoring Committees: 

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