• False Capacity for Lane Drops

    NCDOT Research Project Number: 2003-07

 Executive Summary

  • The practice of discontinuing a through or shared lane at the downstream side of a traffic signal is quite common on North Carolina's streets and highways. That additional lane (sometimes called auxiliary lane) is intended to increase the capacity of the intersection approach. That presumption is true only if drivers will use that lane efficiently both on the upstream and the downstream end of the intersection. Inefficient use of the lane at the upstream end will result in a false calculation of capacity since many drivers may shy away from using the lane as they anticipate the urgent need to merge to the continuous lane at the downstream end. Thus, the auxiliary lane may not produce the full capacity it is meant to provide. Inefficient use of the lane on the downstream end could result in a safety hazard, particularly if the design of the downstream side is inadequate to handle the merging traffic. Thus, these types of lane design have both operational and safety concerns that need to be addressed in the early stage of the design process.

    Currently there is no mechanism by which the degree of lane utilization can be related to the design features and surroundings of the auxiliary lane (i.e., downstream full lane length, taper length, signing, driveways, etc.). The Highway Capacity Manual (HCM) provides default values for lane utilization but these are based strictly on the number and type of approach lanes, and do not take into account the disposition of the lanes(s) downstream of the signal. Further, the HCM deals with lane groups as opposed to individual lanes. The SIDRA model, now used for the analysis of isolated signalized intersections in North Carolina, provides a lane-by-lane utilization factor, which can be calibrated using field data. Regardless of which model is used, the empirical database to calibrate the lane utilization factor is simply non-existent.​

    The purpose of this study is to collect operational and safety-related data at intersection approaches that have auxiliary lanes. The data will be used to study will develop a lane utilization prediction model, which will take into account, among other things, the design features of the downstream lane drop. Also, the safety data will identify those lane design aspects that appear to promote safety and provide a smoother merging process. These two outcomes will result in an appropriate capacity estimation procedure for such design in North Carolina, and in the promotion of safe and efficient designs when such lanes are specified. The results from the study will be summarized in two tables for easy use by NCDOT personnel in capacity analyses and designs.​

    The primary objectives of the research study include the development of:A set of field-verified estimates for the lane utilization factor at a signalized intersection when there is a downstream lane drop.Estimated collision rates for lane drop areas around signalized intersections, including lane drops at the intersection.

    The results from this research will be used by Traffic Engineering, Roadway Design, and other units of the NCDOT. For new designs or existing cases, they will be able to apply proper values of the lane utilization factor when conducting capacity analyses of intersections with auxiliary lanes. The lane utilization factor could be extremely critical in proper functioning of the intersection, changing the estimated capacity by 20 percent or more in some cases. This could be enough to add thousand of hours of delay per year to an intersection with a poorly designed lane drop. Since there are hundreds of these locations around the state, this is a very significant issue for the NCDOT and its motorists. Designers will also be able to estimate the future number of collisions with various lane drop lengths; choosing the proper length could save North Carolina motorists many future collisions.​

    The main product of the research will be two very simple tables: one showing the recommended lane utilization factor for estimates capacity, delay, and level of service and the second showing collision rates. Factors in the tables will likely be roadway geometry, driveway intensity beyond the intersection, right turn percentage, truck percentage, and others. There would be a similar table for collision rates. With minimal training, the results of this research can be applied by an engineer familiar with the Highway Capacity Manual and Highway Capacity Software. A presentation distributing the tables described above, describing how the numbers in the tables were derived, and showing how the numbers from the tables may be substituted into capacity analyses should be adequate. 
Joseph E. Hummer; Nagui M. Rouphail
Gary Faulkner
Mustan Kadibhai, PE, CPM
NC State University

 Related Documents

 Report Period

  • July 2002 - June 2004


  • Complete


  • Planning, Policy, Programming and Multi-modal

 Sub Category

  • Facility Design

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