Designing Crossing Islands for Speed Control and Intersection Safety on Two-Lane Collectors and Arterials

Crossing islands at unsignalized intersections, in addition to their pedestrian crossing safety benefits, can also serve as speed control chicanes by forcing vehicles to make a reverse curve (S-curve). A method is developed for determining the vehicle path radius needed to achieve a target speed, accounting for how driver willingness to accept lateral forces varies with speed, and based on that, the chicane length (and thus, parking setback) needed for a given lane width and island width. The concept of “informal flare” is defined as an approach geometry that allows a left-turning vehicle to wait for a gap in opposing traffic without blocking through traffic behind it. Using informal flares can make it possible to prevent left-turn blockage without sacrificing a crossing island for a left turn lane. Curb continuation lines at median openings are presented as a means to enhance informal flare function. Original data are presented relating speed to vehicle path radius at a crossing island chicane and relating informal flare function (the tendency of through vehicles to bypass a waiting left turner) as a function of a road’s half-width. Geometric analysis shows that intersections with crossing islands can fit on roads with right of way as narrow as 60 ft while still accommodating turning school buses and preventing left turn blockage.

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  • Supplemental Notes:
    • This paper was sponsored by TRB committee AKD80 Standing Committee on Roundabouts and other Intersection Design and Control Strategies.
  • Corporate Authors:

    Transportation Research Board

  • Authors:
    • Furth, Peter G
    • Tahmasebi, Milad
    • Shekari, Sepehr Steve
    • Jackson, Jay
    • Sha, Zhao Howie
    • Alsharif, Yousef
  • Conference:
  • Date: 2021


  • English

Media Info

  • Media Type: Web
  • Features: Figures; References; Tables;
  • Pagination: 20p

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Filing Info

  • Accession Number: 01763831
  • Record Type: Publication
  • Report/Paper Numbers: TRBAM-21-00869
  • Files: TRIS, TRB, ATRI
  • Created Date: Dec 23 2020 11:12AM