Texas A&M Transportation Institute

Transportation Policy Research

Intersection Turn Lanes

System Modification

Introduction

Turns at street and driveway intersections, especially on roads with through traffic, can cause delays and decrease safety. Installing a turn lane can significantly improve the intersection.

Common reasons for installing a turn lane are:

  • Speeds are too high to turn safely to or from a roadway.
  • There is a pattern of rear-end, sideswipe, or weaving crashes when through vehicles conflict with turning vehicles.
  • Drivers have to wait a long time to make a turn.
  • There is a high number of turning vehicles.

Intersection turn-lane improvements could include:

  • Dedicated left-turn and right-turn lanes.
  • Deceleration and acceleration lanes.
  • Space to handle turning traffic away from the through traffic.


Executive Summary

Target Market

Major Street Intersections and Driveways
Major streets and access roads without turn lanes or acceleration/deceleration lanes have congestion from either traffic slowing down to turn or speeding up to join the main lanes. The differences in speed between through traffic and entering/exiting traffic can slow all traffic, cause stop-and-go traffic, and increase collisions.

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How Will This Help?

Turn lanes at intersections greatly reduce crashes by removing stopped vehicles from through traffic, as shown in Table 1.

Research predicts that adding a left-turn lane on one approach of a three-leg stop-controlled intersection will reduce total crashes by 44 percent.1 The developers of the Highway Safety Manual2 used the findings from one study1 along with data from other studies to develop crash modification factors for predicting changes in the number of crashes due to installing a turn lane. The crash modification factors are included in the Highway Safety Manual.

Left-turn lanes also greatly increase the capacity of many roadways. A shared left-turn and through lane has about 40 to 60 percent of the capacity of a standard through lane.3 A review of research on this topic found a 25 percent increase in capacity, on average, for roadways that added a left-turn lane.4

Right-turn lanes typically have less of an impact on roadway capacity than other types of turn strategies because there are fewer limitations on right turns. Though there are not many studies on these impacts, there is a clear connection between the number of vehicles making a right turn in a through traffic lane and the delay it causes to through traffic. Each additional car that must wait for a right turn will increase the delay more than the previous car (see Table 2). At intersections with many right-turn movements, a dedicated right-turn lane separates these cars from through traffic and increases the capacity of the road.

Table 2

 

Implementation Examples

Application Techniques and Principles

Table 2-3 of the TxDOT Access Management Manual provides thresholds for auxiliary lanes. For example, a left-turn lane “may be required if it would provide a benefit to the safety and operation of the roadway.” Chapter 3 of the TxDOT Roadway Design Manual has acceleration and deceleration lengths.

National Cooperative Highway Research Program (NCHRP) Project 3-915 recently used a benefit-cost approach to determine when a left-turn lane would be needed. The steps included simulation to determine:

  • Delay savings from installing a left-turn lane.
  • Crash costs and crash reduction savings determined from safety calculations available in the Highway Safety Manual.
  • Construction costs.

Left-turn lane warrants were developed for rural two-lane highways, rural four-lane highways, and urban and suburban roadways.

NCHRP Project 3-726 used an economic analysis to identify where right-turn lanes at intersections without signals and major driveways would be cost effective. The procedure was used to determine traffic volume combinations that would warrant the need for a right-turn lane.

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Issues

Right of way is the main issue with adding turn lanes or acceleration/deceleration lanes. Increased crossing distance for pedestrians can be another concern.

Changing the current shoulders to usable lanes may require making the pavement stronger and wider. If the shoulder cannot be used, the road will need to be widened. This may require more right of way and result in higher construction costs. Complex, old, or elevated designs will make the project more difficult and costly. Right-of-way limits at intersections may require a complete rebuild or different design.

Project Time Frame

The timeline for adding turn and/or acceleration/deceleration lanes will differ based on:

  • Which method is used.
  • The current road design.
  • The roadway class.

A typical surface road can be converted in a shorter time frame than an elevated section.

Who Is Responsible?

States and cities bear the primary responsibility of installing and maintaining turn lanes. On state roads, the local department of transportation office may take responsibility. On city roads, however, the local government controls the construction and management of turn lanes. In either case, improvements should be coordinated with local citizens and businesses to ensure that the road serves the nearby property.

Cost

The cost of adding a turn lane to a road depends on many factors. A review of the TxDOT Expressway Detailed Letting website7 shows a range of $120,000 to $400,000 in construction costs for left-turn-lane projects, with an average of $250,000.

Data Needs

The basic information that should be used when considering a turn lane includes:

  • The development at that location.
  • The number of lanes on the major roadway (e.g., two, four, or more).
  • The number of approaches (e.g., three streets or four streets).
  • The peak-hour major roadway volume per lane.
  • The peak-hour speed.
  • The number of crashes.
  • The acceptable delay and/or gap (for certain locations).

Other useful information might include:

  • Access to any adjacent developments.
  • Access management considerations.
  • Available right of way.
  • Existing roadway width.
  • Crash history.
  • Consistency with nearby intersections.
  • Sight distance restrictions.
  • Speed differential concerns.
  • Pedestrian traffic.
  • Existing or proposed medians in the area.
  • Available budget.

Intersection Turn Lanes Best Practices

  • Type of location: Major street intersections.
  • Agency practices: Coordinate between planning, design, safety, and operations.
  • Frequency of reanalysis: Annually, during safety reviews.
  • Supporting policies or actions needed: Capability to fund improvements.
  • Complementary strategies: Intersection improvements—pedestrian treatments.

For More Information

American Association of State Highway and Transportation Officials. A Policy on Geometric Design of Highways and Streets. Washington, D.C., 2004.

American Association of State Highway and Transportation Officials. Highway Safety Manual, 1st ed. Washington, D.C., 2010.

Fitzpatrick, K., M. A. Brewer, J. S. Gluck, W. L. Eisele, Y. Zhang, H. S. Levinson, W. von Zharen, M. R. Lorenz, V. Iragavarapu, and E. S. Park. Development of Left-Turn Lane Warrants for Unsignalized Intersections. NCHRP Web Only Report for Project 3-91, 2010.

Fitzpatrick, K., M. D. Wooldridge, and J. D. Blaschke. Urban Intersection Design Guide. Report 0-4365-P2, Texas A&M Transportation Institute, College Station, Texas, 2005.

Florida Department of Transportation. Special Marking Areas, 2006 FDOT Design Standards. July 1, 2005.

Gluck, J., H. S. Levinson, and V. Stover. NCHRP Report 420: Impacts of Access Management Techniques. NCHRP, National Research Council, Washington, D.C., 1999.

Texas Department of Transportation. Access Management Manual. 2011.

Texas Department of Transportation. Roadway Design Manual. 2010.

Transportation Research Board Committee on Access Management. Access Management Manual. National Research Council, Washington, D.C., 2003.

Transportation Research Board. Highway Capacity Manual. National Research Council, Washington, D.C., 2000.

References

  1. Harwood, D. W., K. M. Bauer, I. B. Potts, D. J. Torbic, K. R. Richard, E. R. Kohlman Rabbani, E. Hauer, and L. Elefteriadou. Safety Effectiveness of Intersection Left- and Right-Turn Lanes. Report FHWA-RD-02-089, Federal Highway Administration, U.S. Department of Transportation, 2002.
  2. American Association of State Highway and Transportation Officials. Highway Safety Manual. Washington, D.C., 2010.
  3. Federal Highway Administration Office of Operations. Benefits of Access Management Brochure. http://ops.fhwa.dot.gov/access_mgmt/docs/benefits_am_trifold.htm.
  4. S/K Transportation Consultants, Inc. Access Management, Location, and Design. National Highway Institute Course Number 133078, April 2000.
  5. Fitzpatrick, K., M. A. Brewer, J. S. Gluck, W. L. Eisele, Y. Zhang, H. S. Levinson, W. von Zharen, M. R. Lorenz, V. Iragavarapu, and E. S. Park. Development of Left-Turn Lane Warrants for Unsignalized Intersections. NCHRP Web Only Report for Project 3-91, 2010. http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_w193.pdf.
  6. Potts, I. B., J. F. Ringert, D. W. Harwood, and K. M. Bauer. Operational and Safety Effects of Right-Turn Deceleration Lanes on Urban and Suburban Arterials. In Transportation Research Record: Journal of the Transportation Research Board, No. 2023, National Research Council, Washington, D.C., 2007, pp. 52–62.
  7. Texas Department of Transportation. Detail Letting Schedule for Bastrop County (FY 2010). November 2, 2010. http://www.dot.state.tx.us/insdtdot/orgchart/cmd/cserve/let/2010/bastrop.htm#032103017.