FLH PDDM Chapter 9 - Eastern Federal Lands Highway Division

9.4.B. Intersection Design. (continued)Avoid offset distances wider than 1.5 meters as this gives the appearance of an added lane. Reflective RPMsmay supplement island markings.Raised islands at crosswalk locations require barrier-free access for the handicapped. (See Section 9.4.F.6.)Design approach ends of islands to provide adequate visibility and advance warning of their presence. Islandsshould not cause a sudden change in vehicle direction or speed. Transverse lane shifts should begin far enoughin advance of the intersection to allow gradual transitions. Avoid introducing islands on a horizontal or verticalcurve. When islands on curves are unavoidable, adequate sight distance, illumination, and/or the extensionof the island must be considered.See Part V of the MUTCD and the Green Book for additional design criteria for islands.7. Left-Turn Lanes. A left-turn lane is an auxiliary lane on the left side of a one-directional pavement for useas speed change and storage of left-turning vehicles. Left-turn movements result in more critical trafficconflicts than do right-turn movements. Design left-turn channelization with enough operational flexibilityto function under peak loads and adverse conditions. Left-turn lanes are an economical way to reduce delaysand accidents at intersections.At unsignalized intersections on two-lane highways, use Figure 9-17 for guidance on the need for left-turnlanes. Left-turn lanes are appropriate at locations where accidents involving left- turning vehicles are high.Refer to Figures 9-18 through 9-20 and Table 9-13 to determine the storage length required. The minimumstorage length should be 30 meters. At signalized intersections the left-turn storage length is dependent oncapacity and level of service criteria found in the Highway Capacity Manual.Exhibit 9.8 shows typical left-turn geometrics to accommodate a left-turn lane. Offsets and pavementwidening should be symmetrical about centerline or baseline. Widen on one side only when right-of-way,topography, alignment restrictions, or other circumstances prevent symmetrical widening. See the Green Bookfor additional design guides and for left-turn treatments on multilane facilities.8. Right-Turn Lanes. Right-turn corner designs should allow the design vehicle to turn without encroachingon adjacent lanes, curbs, shoulder edges, or opposing traffic lanes. Exhibit 9.9 shows typical design for thedesign vehicles using a taper at the exit end of the right-turn corner. For a simple radius without the exit taper,the values in Table 9-9 apply, however, this will increase the pavement area. At signalized intersections someencroachment on adjacent lanes of the approach leg is usually acceptable to obtain an adequate radius.Where larger radii than those given in Table 9-9 or Exhibit 9.9 are desirable, compound curves may reducethe need for additional pavement area. See the Green Book for a discussion of compound curves and otherguidelines for corner radius returns.9 - 63