A large number of four-lane undivided roadways exist in the U.S. In many cases, these roadways were originally constructed to serve increasing volumes of traffic within a given right-of-way. Unfortunately, it is now commonly accepted that four-lane undivided roadways experience higher overall crash rates than roadways with other cross sections. It is not surprising, therefore, that many four-lane undivided roadways have experienced an unacceptable degradation of service and safety as traffic volumes have increased. The solution to operational and/or safety problems on four-lane undivided roadways has typically involved the addition of a raised median or two-way-left-turn lane (TWLTL). This type of improvement has accepted mobility and safety benefits, but may also include significant cost and right-of-way impacts. Recently, the implementation of a three-lane cross section (i.e., one lane in each direction and a continuous TWLTL) has been suggested as an alternative, in some cases, to widening four-lane undivided roadways. Guidelines have been proposed to assist with the selection of candidate roadways for four-lane undivided to three-lane conversions. Summarized in this paper is some of the proposed content of these guidelines. Previous research about the conversion of four-lane undivided roadways to three-lane cross sections is discussed, and a number of anecdotal and quantitative before-and-after conversion case studies presented. A qualitative and/or quantitative discussion of several feasibility determination factors is then documented. The factors discussed include roadway function; total traffic volume; turning volumes and patterns; weaving, speed, and queues; collision type and patterns; pedestrian and bike activity; and right-of-way availability and cost. The significance and importance of these factors, and how they are different for four-lane undivided and three-lane cross sections, are evaluated. The results of some ongoing, but preliminary, modeling, simulation, and level of service analyses are also included in the discussion when appropriate. The guidelines produced when this ongoing project is finished should help transportation professionals identify the preferred roadway corridor characteristics for a feasible four-lane undivided to three-lane conversion. It has been concluded, however, that a three-lane cross section may be a viable mitigation measure for a problematic four-lane undivided roadway. The impacts of this type of improvement, therefore, should be analyzed and compared to the impacts of the other feasible alternatives in a detailed engineering study. When properly implemented the conversion of a four-lane undivided roadway to a three-lane cross section can produce acceptable levels of service and also improve safety.

  • Supplemental Notes:
    • The publisher's German name is Forschungsgesellschaft fur Strassen- und Verkehrswesen (FGSV).
  • Corporate Authors:

    Road and Transportation Research Association

    Postbox 50 13 62
    D-50973 Cologne,   Germany 
  • Authors:
    • Knapp, K K
    • Welch, Thomas M
  • Conference:
  • Publication Date: 2000-6


  • English

Media Info

  • Features: Figures; Photos; References; Tables;
  • Pagination: p. 69-80

Subject/Index Terms

Filing Info

  • Accession Number: 00794768
  • Record Type: Publication
  • Report/Paper Numbers: FGSV 002/67
  • Files: TRIS
  • Created Date: Jun 23 2000 12:00AM