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Rh Gradients on ramps.—The gradient on ramps shall not exceed 6 percent on upgrades and 8 percent on downgrades.

Sight distance at ramps and connections.—On all ramps and connections the combination of grade, vertical curves, alinement, and clearances of lateral and corner obstructions to vision shall be such as to provide sight distance along such ramps and connections and from their terminal junctions along the interregional highway and intersecting road consistent with the probable speeds of vehicle operation.

At all grade intersections on rural sections of the interregional system, vehicles approaching on the intersecting highways shall be required, before crossing the interregional highway, to come to a complete stop at a point off the through-traffic lanes of the interregional highway. From such point of stopping, the sight distance in each direction along the interregional highway and from the rear along the intersecting highway, shall be not less than the safe stopping distance corresponding to the probable speed of traffic on each high- way, as hereinbefore recommended.

Access for busses.—Bus stops shall be prohibited on all rural sections of the interregional system. Access connections for bus stops off the interregional highway shall be designed to the same standards as other access connections.

Access for roadside businesses.—Roadside businesses, such as parking areas, gasoline filling stations, restaurants, etc., shall be prohibited from fronting directly on rural sections of the interregional system. Access connections for such businesses off the interregional highway shall be designed to the same standards as other access connections.

V. Tunnels.

Tunnels on rural sections of the interregional highway system shall accommodate, if financially feasible, the same number and width of traffic lanes that are provided on the highway approaching the tunnel. Tunnels on two-lane highways shall provide space for a 2-foot flush median strip between the two lanes, and shall have an over-all width not less than 10 feet greater than the pavement or surface width of the highway approaching the tunnel, to provide for the 2-foot center separation and not less than 4-foot side clearances beyond the edges of the pavement.

Tunnels on divided highways shall be constructed to accommodate the same number of lanes as the approaches, in either a single or twin bore. In the former case the lanes for traffic in opposite directions shall be separated by a raised but mountable median strip not less than 4 feet wide; and the over-all width of the tunnel shall provide space for this strip and two lateral clearances of not less than 4 feet, in addition to the aggregate width of traffic lanes of the same number and width as on the highway approaching the tunnel. In the case of twin bores, the over-all width of each bore shall be not less than 8 feet greater than the total width of the lanes to be accommodated, as provided on the highway approaching the tunnel.

The clear height provided in tunnels shall be not less than 14 feet across the entire paved width.

Standards of gradient and curvature for tunnels shall be identical with those previously recommended for divided highways.

Wherever their length requires, all tunnels shall be-artificially ventilated and lighted in such manner as to provide amply safe conditions