FLH PDDM Chapter 9 - Eastern Federal Lands Highway Division

9.4.B. Intersection Design. (continued)The multileg intersection has more than four intersecting approach legs and can form severalconfigurations. For purposes of this discussion, the rotary intersection is considered to be a multileg.These intersections have visibility problems and poor turning angles, confuse the road user, and aredifficult to sign, mark, and signalize. This type of intersection occurs when a highway diagonallycuts across a street grid system or when more than four approach legs intersect. The multilegconfiguration is not appropriate for new highways, and the reconstruction of existing multilegs to thefour-leg type intersection is very desirable.The “roundabout” intersection, a small radius version of a rotary intersection/traffic circle hasworked well on some low-speed routes as replacements for intersections with 4-way stops. It hasbeen very successful in reducing accidents, but failures have shown that site selection is critical tosuccessful performance.2. Design Vehicle. Design vehicles have selected dimensions and operating characteristics. Each representsa class of vehicles that establish design controls for specific conditions.The design vehicle for any intersection depends on the roadways involved, the location of the intersection, andthe types and volume of vehicles using the intersection. Table 9-9 provides a guide to determine the designvehicle appropriate for various intersections.Design an intersection so the design vehicle can make all turning movements without encroaching on adjacentlanes, opposing lanes, curbs, or shoulders. Using a taper at the exit end of the right-turn corner will reducethe radius and the pavement area. For the recommended right-turn corner design see Section 9.4.B.8, Right-Turn Lanes.Table 9-9Intersection Design VehicleDesign VehiclesRadius (meters)Intersection Type Desirable Minimum Desirable MinimumJunction of Major Truck Routes WB-19 WB-19 35 30Junction of State Routes WB-15 WB-12 30 23Ramp terminals WB-15 WB-12 30 23Other Rural WB-12 SU 23 17Urban Industrial WB-12 SU 23 17Urban Commercial SU P 17 11Residential SU P 17 11Note:P = Passenger car, including light delivery trucks.SU = Single unit truck.WB - 12 = Semitrailer truck, overall wheelbase of 12 meters.WB - 15 = Semitrailer truck, overall wheelbase of 15 meters.WB - 19 = Semitrailer truck, overall wheelbase of 19 meters.9 - 59