intersecting highway also shall be designed as one-way roads separated by suitable channelizing islands. Entrances and exits shall be located at sufficient distances from the grade-separating structure to provide sight distances adequate for safety under the conditions of vehicular speed to be anticipated.
All ramps and connections shall be designed to enable vehicles to leave and enter the through-traffic lanes of each highway at 0.7 of its design speed, except where “stop” control is necessary. On all four-lane sections of the system, on all two-lane sections expected to carry an average daily traffic of more than 2,000 vehicles, and wherever feasible on all other two-lane sections, the curvature of ramps and connections shall preferably not exceed 45 degrees (radius approximately 125 feet), and under no conditions shall exceed 70 degrees (radius approximately 80 feet). All curves shall be eased by transition or compounding.
Widths of pavements and shoulders and side slopes.—All ramps and connections shall have a width of at least 16 feet if designed for one-lane operation, and at least 26 feet if designed for two-lane operation. Widths greater than these minima shall be provided on sections of ramps and connections of sharp curvature.
A shoulder at least 6 feet and preferably 8 feet wide shall be provided along the right side of all ramp pavements (right in the direction of traffic movement).
Side slopes on ramps shall be not steeper than 1 foot measured vertically to 2 feet measured horizontally, and shall be rounded at the top and bottom to merge by curves of natural appearance with the adjacent land slopes or shoulders.
Added space for turning maneuvers.—All rural sections of the system shall be so designed, at the approach to entrances and exits on the interregional highway, as to provide space outside the through-traffic lanes of the interregional highway for emerging vehicles to decelerate and entering vehicles to accelerate, and, in general, to maneuver as required so that they may safely emerge from or merge with the through traffic stream. The added space may take the form of a taper, a pavement gradually increasing in width, or a taper combined with a lane of full added width. Tapers shall be smoothly alined and of a length consistent with probable speed of travel. Where a full width of added lane is required, it shall be at least 10 feet wide. If deemed necessary and feasible, similar provision shall be made on the intersecting highway.
Where an exit to an inner loop is provided on one side of a grade-separating structure and an entrance from an inner loop is provided on the opposite side of the structure, an added lane shall be carried over the bridge or through the underpass to connect both inner loops and serve as added space for the maneuvers of entering and leaving the interregional highway.
At exits from a through-traffic lane, added width of pavement and a taper shall be provided beyond the nose at the fork to enable vehicles which start to turn off to return safely and conveniently to the through-traffic lane if desired. A curb of high visibility should. be used around the nose at the fork and along these widened pavements.
All pavement surfaces of ramps and connections and all added pavement width provided for maneuvering shall contrast in color and preferably in texture with the pavement of the through-traffic lanes.
