Chapter 4 - Elements of Design

4.2. Horizontal Alignment

4.2.5. Transition in Number of Lanes

Instances where the number of lanes on a roadway is transitioning fall into two categories – lane additions and lane drops. Lane drops induce a merge situation. Adequate distance for drivers to perform the merge maneuver should thus be provided. Lane additions that do not involve a shift of the mainline lanes may be accomplished in a much shorter distance.

Lane Drops

Lane drops can occur in many situations on all types of roadways, such as:

• mainline lane drop due to traffic drop off

• mainline lane drop to meet lane balance requirements (limited access)

• mainline lane drop due to transition to non-widened section, etc.

• end of auxiliary lane

• end of collector-distributor (cd) system

• end of climbing lane

• ramp merges on limited access facilities

With three exceptions, lane drops (or merges) for the situations listed above should be designed based on the minimum convergence tapers provided in Section 4.2.4. Lane Width Transitions and Shifts.

Exception 1 – For lane drops and merges on high-speed Limited Access facilities, where design year mainline peak hour traffic rates exceed 1,550 vehicles per lane (LOS C), the convergence ratio shall be:

L = 2 * W * s  

             

Where: 

L = distance needed to develop widening (ft)

W = width of lane shift (ft)
s = design speed (mph)

 

Exception 2 – Certain situations require the use of horizontal curves and possibly superelevation in association with lane reductions. An example of this would be tie-ins being constructed on projects between a proposed four-lane section (with 44-ft. median) and a two-lane existing section. In these situations, a lane shall first be dropped using the taper rates specified in Section 4.2.4. Lane Width Transitions and Shifts, while still on the four-lane section (in advance of the crossover). Once the lane reduction has been attained, the tie-in to the two-lane section shall be accomplished with a tie-in using AASHTO horizontal curves and superelevation rates appropriate for the design speed of the facility. If possible, the curves associated with the tie-in should be no sharper than 1 degree.

Exception 3 - If a ramp merge occurs on a significant upgrade, the speed differential of a truck or bus merging into traffic shall be evaluated. In general, if the mainline grades exceed 3% (upgrade in merge), the convergence ratio shall be:

L = 2 * W * s 

 

Where:        

L = distance needed to develop widening (ft)

W = width of lane shift (ft)
s = design speed (mph)

 

 General Rules on Lane Drops

• lane drops on limited access facilities should occur at exits
• lane drops on limited access facilities should occur on the outside lanes
• upon departing an intersection, a lane (to be dropped) should be maintained for a minimum of 800-ft. from the intersection before initiating the lane drop
• tapers associated with multiple, successive lane drops on the mainline should be separated by a minimum 1,000-ft. tangent section

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