4.8.1 Grades

The effects of rate and length of grade are more pronounced on the operating characteristics of trucks than on passenger cars and thus may introduce undesirable speed differentials between the vehicle types. The term
“critical length of grade”
 is used to indicate the maximum length of a specified ascending gradient upon which a loaded truck can operate without an unreasonable reduction in speed (commonly 10 mph).
Figure 4-4
shows the relationship of percent upgrade, length of grade, and speed reduction for a representative truck (200-lb/hp weight/power ratio) with an entering speed of 70 mph. For additional entry speeds and/or design vehicles see
AASHTO’s A Policy on Geometric Design of Highways and Streets
.
Where critical length of grade is exceeded for any type of two-lane facility, climbing lanes should be considered as discussed in the HCM.
When the critical length of grade values shown in
Figure 4-4
are exceeded, consider the effect on the upstream traffic of adding climbing lanes (i.e., two-lane facilities on direct connectors should be considered when a one-lane facility would significantly reduce the LOS).
Critical Lengths of Grade for Design, Assumed Typical Heavy Truck of 200-lb/hp, Entering Speed = 70 mph ( click in image to see full-size image)
Figure 4-4: Critical Lengths of Grade for Design, Assumed Typical Heavy Truck of 200-lb/hp, Entering Speed = 70 mph
Source: AASHTO A Policy on Geometric Design of Highways and Streets
Table 4-11: Maximum Grades
2
Functional Classification
Type of Terrain
Design Speed
(mph)
15
20
25
30
35
40
45
50
55
60
65
70
75
80
Rural Town, Suburban, Urban and Urban Core
Local
1
All
8
8
8
8
8
8
8
Collector
3
Level
9
9
9
9
9
9
8
7
7
6
Rolling
12
12
12
11
10
10
9
8
8
7
Arterial
3
Level
8
7
7
6
6
5
5
Rolling
9
8
8
7
7
6
6
Freeway
4
Level
4
4
3
3
3
3
3
Rolling
5
5
4
4
4
4
4
Rural
Local
Level
9
8
7
7
7
7
7
6
6
5
Rolling
12
11
11
10
10
10
9
8
7
6
Collector
3
Level
7
7
7
7
7
7
6
6
5
Rolling
10
10
9
9
8
8
7
7
6
Arterial
Level
5
5
4
4
3
3
3
3
3
Rolling
6
6
5
5
4
4
4
4
4
Freeway
4
Level
4
4
3
3
3
3
3
Rolling
5
5
4
4
4
4
4
Notes:
  1. . Up to 15% maximum can be used in very constrained conditions. Flatter gradients should be used where practical and grades less than 5% are Preferable.
  2. The maximum grades shown may be increased by 1% where there are one-way downgrades that are less than 500-ft in length. In these cases, a design exception would not be required.
  3. Where heavy truck traffic is present (over 10%) it is recommended to use a max grade of 5% in level terrain and 6% in rolling terrain.
  4. See 15.7.3 for ramp and direct connector criteria.
Table 4-11
summarizes the maximum grade controls in terms of design speed. See
Section 4.6
for defining level, rolling, and mountainous terrain conditions. Generally, the
maximum design grade should be used infrequently rather than as a value to be used in most cases.
 However, for certain cases such as urban freeways, a maximum value may be applied in blanket fashion on interchange and grade separated approaches.
Flat or level grades on uncurbed pavements are satisfactory when the pavement is adequately crowned to drain the surface water laterally and superelevation transitions are not co-located (see
Section 4.9
).
When roadside ditches are required and follow the grade of the roadway, the roadway profile grade should seldom be less than 0.5 percent for unpaved ditches and 0.25 percent for lined channels.
When these roadway profile grade conditions are not met, special ditch profiles, independent of the roadway profile may be required to ensure positive ditch drainage.
For curbed pavements, a minimum roadway profile grade of 0.3 percent should be provided to facilitate surface drainage.
 Joint analyses of rainfall frequency and duration, longitudinal grade, cross slope, curb inlet type, and spacing of inlets or discharge points are usually required so the width of water on the pavement surface during likely storms does not unduly interfere with traffic. Criteria for water ponding for various functionally classified roadways are contained in TxDOT’s
Hydraulic Design Manual (HDM)
.