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Adding Lanes
When additional lanes are developed, such as for passing lanes, climbing lanes, additional lanes at intersections, or in other circumstances, they should be developed with a 15:1 taper ratio, as shown in Figure 1. This section discusses adding, dropping, and redirecting through lanes. Taper ratios for right- and left-turn lanes are discussed in Section 6A-1.
Dropping Lanes
When dropping a through lane, the minimum length of taper can be determined by the following formulas:\(^{1}\)
\(L = \frac{WS^2}{60}\) for speeds of 40 mph or less
\(L = \frac{WS^2}{155} \text{ for speeds of 70 km/h or less}\)
\(L = S \times W \text{ for speeds of 40 mph or more}\)
\(L = 0.62 \times S \times W \text{ for speeds of 70 km/h or more}\)
where:
L = minimum length of taper.
S = posted speed limit or 85th percentile speed.
W = width of lane to be dropped or redirection offset.
Preferably, taper ratios should be evenly divisible by 5 (15:1, 20:1, etc.). Calculations that result in odd ratios should be rounded up to the next increment of 5. Table 1 utilizes the formulas to determine the appropriate taper ratios for dropping a 12-foot (3.6-meter) wide lane. The ratio remains constant for a given design speed, while the length varies with the lane width.
\(^{1}\)FHWA, Manual on Uniform Traffic Control Devices, Washington, D.C., U.S. Government Printing Office, 2000.
Table 1: Length and Taper Ratio for Dropping 12-foot (3.6-meter) Lane
Redirecting Lanes
The procedure for determining minimum taper ratios for redirecting through lanes is the same as shown in Table 1 for lane drops; however, for design speeds over 45 mph (70 km/h), the use of reverse curves rather than tapers is recommended. Figure 2 below illustrates a taper for redirecting through lanes and Figure 3 illustrates redirecting through lanes using reverse curves. Section 2D-1 of this manual provides more information regarding reverse curves.
