1360.04(4)(b) Gap Acceptance
For parallel on-connections, provide the minimum gap acceptance length (Lg) to allow entering motorists to
evaluate gaps in the freeway traffic and position their vehicles to use the gap. The length is measured beginning
at the point that the left edge of traveled way for the ramp intersects the right edge of traveled way of the main
line to the ending of the acceleration lane (see Exhibit 1360-16 and Exhibit 1360-17). The gap acceptance length
and the acceleration length overlap, with the ending point controlled by the longer of the two.
Chapter 1360
Interchanges
WSDOT Design Manual M 22-01.20
Page 1360-14
September 2021
Exhibit 1360-10 Acceleration Lane Length
Tapered On-Connection
Parallel On-Connection
Highway Design
Speed (mph)
Ramp Design Speed (mph)
0
15
20
25
30
35
40
45
50
60
70
30
180
140
35
280
220
160
40
360
300
270
210
120
45
560
490
440
380
280
160
50
720
660
610
550
450
350
130
55
960
900
810
780
670
550
320
150
60
1200
1140
1100
1020
910
800
550
420
180
65
1410
1350
1310
1220
1120
1000
770
600
370
70
1620
1560
1520
1420
1350
1230
1000
820
580
210
80
2000
1950
1890
1830
1730
1610
1380
1200
970
590
210
Minimum Acceleration Lane Length (ft)
Highway Design
Speed (mph)
Grade
Upgrade
Downgrade
Ramp Design Speed
All Ramp Design
Speeds
20
30
40
50
40
3% to less
than 5%
1.3
1.3
0.70
45
1.3
1.35
0.675
50
1.3
1.4
1.4
0.65
55
1.35
1.45
1.45
0.625
60
1.4
1.5
1.5
1.6
0.60
70
1.5
1.6
1.7
1.8
0.60
40
5% or more
1.5
1.5
0.60
45
1.5
1.6
0.575
50
1.5
1.7
1.9
0.55
55
1.6
1.8
2.05
0.525
60
1.7
1.9
2.2
2.5
0.50
70
2.0
2.2
2.6
3.0
0.50
Adjustment Factors for Grades Greater Than 3%
Note: Lane widths are shown for illustrative purposes. Determine lane widths based on Exhibit 1360-6.
Chapter 1360
Interchanges
WSDOT Design Manual M 22-01.20
Page 1360-15
September 2021
1360.04(4)(c) Single-Lane On-Connections
Single-lane on-connections may be either tapered or parallel. Design single-lane tapered on-connections as
shown in Exhibit 1360-15 and single-lane parallel on-connections as shown in Exhibit 1360-16.
1360.04(4)(d) Two-Lane On-Connections
For two-lane on-connections, the parallel is desirable. Design two-lane parallel on-connections as shown in
Exhibit 1360-17. A capacity analysis will normally be the basis for determining whether a freeway lane or an
auxiliary lane is to be provided.
Justify the use of a two-lane tapered on-connection. Design two-lane tapered on connections in accordance with
Exhibit 1360-18.
1360.04(5) Off-Connections
Off-connections are the paved areas at the beginning of an off-ramp, connecting it to a main lane of a freeway.
They have two parts: a taper for maneuvering out of the through traffic and a deceleration lane to slow to the
speed of the first curve on the ramp. Deceleration is not assumed to take place in the taper.
Off-connections are either tapered or parallel. The tapered is desirable because it fits the normal path for most
drivers. When a parallel connection is used, drivers tend to drive directly for the ramp and not use the parallel
lane. However, when a ramp is on the outside of a curve, the parallel off-connection is desirable. An advantage
of the parallel connection is that it is narrower at the beginning of the ramp.
1360.04(5)(a) Deceleration Lane
Provide the minimum deceleration lane length given in Exhibit 1360-11 for each design speed for all off-ramps.
Also, provide deceleration lane length to the end of the anticipated queue at the ramp terminal. When the
average grade of the deceleration lane is 3% or greater, multiply the distance from the Minimum Deceleration
Lane Length table by the factor from the Adjustment Factor for Grades table.
For existing ramps that do not have significant crashes in the area of the connection with the freeway, the
freeway posted speed may be used to calculate the deceleration lane length for Preservation projects. If
corrective action is indicated, use the freeway design speed to determine the length of the deceleration lane.
The deceleration lane is measured from the point where the taper reaches the selected ramp lane width to the
first point designed at each ramp design speed (usually the PC of the first curve for each design speed). Curves
designed at higher design speeds may be included as part of the deceleration lane length.
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