Chapter 1360
Interchanges
WSDOT Design Manual M 22-01.20
Page 1360-11
September 2021
Exhibit 1360-8 Lane Balance
3
3
3
4
4
2
2
3
3
3
2
2
3
3
2
2
2
Undesirable: Lane balance, but no compliance with basic number of lanes.
Undesirable: Compliance with basic number of lanes, but no lane balance.
Desirable: Compliance with both lane balance and basic number of lanes.
1360.04(2) Main Line Lane Reduction
The reduction of a basic lane or an auxiliary lane may be made at a two-lane exit or may be made between
interchanges. When a two-lane exit is used, provide a recovery area with a normal acceleration taper. When a
lane is dropped between interchanges, drop it 1,500 to 3,000 feet from the end of the acceleration taper of the
previous interchange. This allows for signing but will not be so far that the driver becomes accustomed to the
number of lanes and will be surprised by the reduction (see Exhibit 1360-9).
Reduce the traveled way width of the freeway by only one lane at a time.
1360.04(3) Sight Distance
Locate off-connections and on-connections on the main line to provide decision sight distance for a
speed/path/direction change as described in Chapter 1260.
Chapter 1360
Interchanges
WSDOT Design Manual M 22-01.20
Page 1360-12
September 2021
Exhibit 1360-9 Main Line Lane Reduction Alternatives
Lane between closely spaced ramps
dropped after single-lane off-connection
(lane balance for weaving)
500 to 1,000 ft
Lane between closely spaced ramps
dropped at single-lane off-connection
(lane imbalance for weaving)
Lane dropped at two-lane off-connection
500 to 1,000 ft
Lane dropped within interchange
Lane dropped after interchange
1,500 to 3,000 ft
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