Chapter 1360
Interchanges
WSDOT Design Manual M 22-01.20
Page 1360-6
September 2021
1360.02(5) Drainage
Avoid interchanges located in proximity to natural drainage courses. These locations often result in complex and
unnecessarily costly hydraulic structures. The open areas within an interchange can be used for stormwater
detention facilities.
1360.02(6) Uniformity of Exit Pattern
While interchanges are of necessity custom-designed to fit specific conditions, it is desirable that the pattern of
exits along a freeway have some degree of uniformity. From the standpoint of driver expectancy, it is desirable
that each interchange have only one point of exit, located in advance of the crossroad.
Exhibit 1360-3 Minimum Ramp Connection Spacing
On-On or Off-Off
Off-On
Turning Roadways
On-Off (Weaving)
L
L
L
L
[1]
Freeway
C-D Road
Freeway
C-D Road
System
[2]
Interchang
e
Service
[3]
Interchang
e
A
B
C
D
1,000
800
500
400
800
600
200
0
160
0
160
0
100
0
Gore nose
L =
Minimum distance in feet from gore nose to gore nose.
A =
Between two interchanges connected to a freeway: a system interchange[2] and a service
interchange.[3]
B =
Between two interchanges connected to a C-D road: a system interchange[2] and a service
interchange.[3]
C =
Between two interchanges connected to a freeway: both service interchanges.[3]
D =
Between two interchanges connected to a C-D road: both service interchanges.[3]
Notes:
These values are based on operational experience, need for flexibility, and signing. Check them in accordance
with Exhibit 1360-12 and the procedures outlined in the Highway Capacity Manual, and use the larger value.
[1]
With justification, these values may be reduced for cloverleaf ramps.
[2] A system interchange is a freeway-to-freeway interchange.
[3] A service interchange is a freeway-to-local road interchange.