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59 Description Alternative intersections and interchanges reroute one or more turning movements, often left turns, from their normal location at a conventional four-leg intersection (1). A number of alternative intersection forms create secondary junctions to accommodate the rerouted move- ments (1, 2). Alternative intersections are designed to reduce delay by reducing the number of signal phases required and improve safety by reducing the number of conflict points (1). Tables 42 through 44 follow. Table 42 lists and describes a number of alternative intersection and interchange forms. Quantitative Analysis Methods Operations Chapter 23 in the HCM6 (1) provides methods for analyzing the vehicular operations of diverging diamond interchanges, displaced left turns, median U-turns, restricted crossing U-turns, and conventional interchange areas. Chapters 19 and 20 in the HCM6 can be used to analyze conventional intersection forms with rerouted turning movements. When comparing intersection forms, the overall travel time by movement through the entire system of main and secondary junctions should be compared rather than simply control delay at the main intersection. C H A P T E R 1 2 Alternative Intersections and Interchanges Source: Photograph provided by Google Earth.
60 Guide for the Analysis of Multimodal Corridor Access Management Intersection Type Description Diagram Single Intersection Forms Indirect left turn (jughandle) Major street left turns at a three-leg intersection exit right and form a fourth leg. Minor street left turns are prohibited (2). Jughandle with far- side ramp Major street left turns pass through the intersection, use a loop ramp to merge onto the side street, and pass through the intersection again (2). Flyover A major or a minor street left-turn movement is grade separated (3). Multiple Junction Forms Jughandle with near-side ramps All major street turns exit prior to the intersection and make their turns at a secondary intersection with the cross street (2). Jughandle with median U-turn Major street left turns pass through the intersection, exit right onto a jughandle ramp to make a U-turn, and then make a right turn at the intersection (2). Quadrant intersection Left turns at the main intersection are redirected to connecting roadways via a series of right turns, left turns, or a mix of both (2). Median U-turn (Michigan U-turn) Major street left turns pass through the intersection, make a U-turn at a directional median crossover, and return to the intersection to turn right. Minor street left turns make a right turn followed by a U-turn (1). Restricted crossing U-turn (superstreet) Major street left turns and minor street left turns and through movements are redirected to directional U-turn crossovers on the major street beyond the intersection (1). Table 42. Alternative intersection and interchange forms.
Alternative Intersections and Interchanges 61 Intersection Type Description Diagram Interchange Forms Diverging diamond interchange Through traffic on the arterial is crossed over to the opposite side prior to the interchange and crossed back after the interchange, allowing free-flowing turning movements (1). Freeway frontage roads Traffic exiting the freeway merges onto a one-way frontage road before turning onto the crossroad. Traffic to the freeway uses a left-hand ramp from the frontage road (3). Displaced left turn (continuous flow intersection) Left turns on one or both streets are crossed over to the far side of opposing traffic prior to reaching the main intersection (1). Grade-separated intersection The major street goes under or over the minor street. The minor street is accessed via ramps from the major street (3). Table 42. (Continued). Access Management Technique Performance Trends and Documented Performance Relationships Operations Safety Implement indirect left turns at existing intersection. â â â Â Â â Â â Â Â Â Redirect left turns via connecting roadways. â Â Â Â Â Â Â Â Â Â Â Redirect left turns via flyover. â â â Â Â â Â â Â Â Â Redirect left turns via U-turns. â â â Â â â Â Â Â Â Â Â Redirect left turns by displacing travel lanes. â Â Â â Â Â â â Â Â Â Build interchange at activity center or major intersection. â â â Â Â â Â â Â Â Â Modify freeway ramps to improve access. â Â Â Â Â Â Â Â Â Â Build freeway frontage roads. â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â ÂÂ ÂÂ Â ÂÂ Â Â Table 43. Multimodal operations and safety performance summary.
62 Guide for the Analysis of Multimodal Corridor Access Management Chapter 23 in the HCM6 provides a measureâexperienced travel timeâfor performing such a comparison (1). No methods have been developed yet for assessing pedestrian or bicycle performance at alter- native intersections. Estimates of vehicular speed can be used as starting points for estimating bus and truck speeds. Safety Because of the relative rarity and in some cases relative newness of alternative intersection and interchange forms, few quantitative safety studies have been performed. Some of the studies that have been performed are simple before-and-after studies that did not correct for potential biases Mode Operations Safety Typically increase capacity and reduce major street vehicular delay at the main intersection (1). May increase overall travel time for rerouted turning movements (1). Clear signing is required to indicate the required lane positioning for rerouted turning movements, as well as to prevent wrong- way movements (4â8). Multiple junction forms must consider the interactions of the main and secondary junctions (2). Restricted crossing U-turns allow separate progression bands for each direction (2). Typically decrease the total number of conflict points, although not necessarily the conflicting volume (4â8). Quantitative safety information not available for many alternative intersection forms. Typically allow shorter traffic signal cycle lengths and thus reduced pedestrian delay for a given crossing stage (4â8). Some intersection forms require multiple-stage pedestrian crossings (6, 8). Signalized U-turn crossovers offer the potential for signalized midblock pedestrian crossings without affecting traffic progression (7). Often decrease the total number of vehicleâ pedestrian conflict points (4â7). Depending on intersection type, traffic at crossings may approach from an unexpected direction (5). Intersection forms with channelized right turns require special attention to the pedestrian crossings of the right turns (6). See Chapter 11. Typically allow shorter traffic signal cycle lengths and a greater percentage of green time for movements, thus reducing delay (7). Can be traversed in the same ways as conventional intersectionsâas a vehicle, as a pedestrian, or by making a two-stage left turn (6). Restricted crossing U-turns pose a challenge for bicyclists affected by rerouted movements (8). Typically decrease the total number of vehicleâ bicycle conflict points (4â8). Ramp and channelized right-turn merge and diverge points require special attention (6). Similar to motor vehicles and trucks. Median U-turns can accommodate a bus or rail transit guideway in the roadway median (7). Can be challenging to place bus stops at displaced left turns (6). Center island of diverging diamond interchanges can be used for bus stops serving transfers between arterial and freeway-based transit routes (5). No documented effect beyond that generally observed for motor vehicle and pedestrian traffic. Intersection forms requiring U-turns may need a loon (widened pavement area on the edge of the roadway) to accommodate larger vehiclesâ turning radii (7, 8). No documented effect beyond that generally observed for motor vehicle traffic. Note: Given the range of alternative intersection forms and the availability of information specific to each type (2â8), only general trends and the most important aspects of selected alternative intersection forms are in the table. Table 44. General trends associated with alternative intersections and interchanges.
Alternative Intersections and Interchanges 63 such as traffic volume changes and regression to the mean (5). Several FHWA publications (4â8) summarize available knowledge about alternative intersection and interchange safety. An FHWA synthesis of 25 studies of median U-turn intersections (9) found the following reductions in crash rates at Michigan U-turns relative to conventional signalized intersections: â¢ Corridor-wide, all crash severities: 14% â¢ Intersections, all crash severities: 16% â¢ Intersections, injury crashes: 30% Additional Information â¢ Chapters 1, 2, and 14 in this guide. â¢ Access Management Manual, Second ed.: Sections 20.2.8 and 20.5. â¢ FHWA alternative intersection informational reports and guides (4â8). References 1. Highway Capacity Manual: A Guide for Multimodal Mobility Analysis, 6th ed. Transportation Research Board, Washington, D.C., 2016. 2. Williams, K. M., V. G. Stover, K. K. Dixon, and P. Demosthenes. Access Management Manual, Second ed. Transportation Research Board of the National Academies, Washington, D.C., 2014. 3. Gluck, J., H. S. Levinson, and V. Stover. NCHRP Report 420: Impacts of Access Management Techniques. Transportation Research Board of the National Academies, Washington, D.C., 1999. 4. Hughes, W., R. Jagannathan, D. Sengupta, and J. Hummer. Alternative Intersections/Interchanges: Informational Report. Report FHWA-HRT-09-060. Federal Highway Administration, Washington, D.C., April 2010. 5. Schroeder, B., C. Cunningham, B. Ray, A. Daleiden, P. Jenior, and J. Knudsen. Diverging Diamond Interchange Informational Guide. Report FHWA-SA-14-067. Federal Highway Administration, Washington, D.C., Aug. 2014. 6. Steyn, H., Z. Bugg, B. Ray, A. Daleiden, P. Jenior, and J. Knudsen. Displaced Left Turn Informational Guide. Report FHWA-SA-14-068. Federal Highway Administration, Washington, D.C., Aug. 2014. 7. Reid, J., L. Sutherland, B. Ray, A. Daleiden, P. Jenior, and J. Knudsen. Median U-Turn Informational Guide. Report FHWA-SA-14-069. Federal Highway Administration, Washington, D.C., Aug. 2014. 8. Hummer, J., B. Ray, A. Daleiden, P. Jenior, and J. Knudsen. Restricted Crossing U-Turn Informational Guide. Report FHWA-SA-14-070. Federal Highway Administration, Washington, D.C., Aug. 2014. 9. Jagannathan, R. Synthesis of the Median U-Turn Intersection Treatment, Safety, and Operational Benefits. Publication FHWA-HRT-07-033. Federal Highway Administration, Washington, D.C., 2007.