15.7.2 General Ramp Design Considerations

15.7.2.1 Design Speed
All ramps and connections should be designed to enable vehicles to leave and enter the traveled way of the freeway at 85 percent (desirable) to 70 percent (usual) of the freeway's design speed, rounded up to the nearest 5 mph increment, and limiting the speed differential to 10 mph on the upper range and 20 mph for the mid-range. The upper and mid-range design speeds are not always practical (e.g., constrained ROW or retrofit to existing frontage roads, etc.) and lower design speeds may be selected upon review from Design Division, but they should not be less than the lower range presented in
Table 15-1.
However, every effort should be made to meet the desirable ramp/connector design speed. These speeds do not pertain to ramp terminals, which should be properly transitioned and provided with speed-change facilities adequate for the highway speed involved.
Where the ramp joins a frontage road, the design speed over the length of the ramp may vary, with the portion of the ramp closer to the frontage road being designed to the frontage road speed (i.e., the lower speed).
The independent design speed of the ramp should normally be assumed to be from the end of one physical nose to the beginning of the opposite physical nose.
 See
Figure 15-22.
for the definition of gore area characteristics.
The design speed for a ramp should not be less than the design speed on the intersecting frontage roads. Refer to
AASHTO's A Policy on Geometric Design of Highways and Streets
for additional guidance on the application of the ranges of ramp design speed shown in
Table 15-1.
. Refer also to
NCHRP Report 313 Selecting Ramp Design Speeds, Vol 1 and 2
Guides for more detailed analysis.
Table 15-1: Guide Values for Ramp/Connection Design Speed as Related to Highway Design Speed
1
Ramp/Connector Design Speed
2
 (mph)
Highway Design Speed (mph)
30
35
40
45
50
55
60
65
70
75
80
Upper Range (85%)
25
30
35
40
45
50
50
55
60
65
70
Mid- Range (70%)
20
25
30
30
35
40
45
45
50
55
60
Lower Range (50%)
15
20
20
25
25
30
30
35
35
40
40
Notes:
1. For corresponding superelevation methodology see Table 4-3. 2.
2.Upper-range values of design speed generally are not attainable on loop ramps. For highway design speeds above 50 mph, loop design speed should be no less than 20 mph.