4.7.4 Superelevation Rates on Low-Speed Rural Town, Suburban, Urban and Urban Core Facilities

Although superelevation is advantageous for traffic operations, various factors often combine to make its use impractical in many rural town, suburban, urban, and urban core areas. These factors include the following:
  • Wide pavement areas;
  • Surface drainage considerations;
  • Frequency of cross streets and driveways; and
  • Need to meet the grade of adjacent property.
For these reasons, horizontal curves on low-speed rural town, suburban, urban, and urban core facilities are frequently designed with normal crown. The centripetal acceleration, in this case, is counteracted solely with side friction. The term
“normal crown”
 (NC) represents an equal downward pavement cross-slope, typically 2 percent, on each side of the axis of rotation.
Low-speed rural town, suburban, urban, and urban core facilities should be designed using NC, such that superelevation is not necessary where practical. This is accomplished by using the negative e-values from
Table 4-4
.
However, when superelevation is needed, a maximum superelevation rate of 4 percent should be used. This is accomplished by using the positive e-values from
Table 4-4
.
Table 4-3: Superelevation Methodology Summary
1
Low-Speed
(≤ 45 mph)
Intermediate-Speed
(50 – 60 mph)
High-Speed
(65 mph and greater)
Urban
All Functional Classifications
(Excluding Freeway Mainlanes,
Ramps and Direct Connectors)
Method 2
Table 4-4
Method 5
4%, 6% or 8% emax
Table 4-5, Table 4-6 or Table 4-7
Method 5
6% or 8% emax
Table 4-6 or Table 4-7
Rural
All Functional Classifications
Method 5
6% or 8% emax
Table 4-6, or Table 4-7
Urban or Rural
Freeway Mainlanes, Ramps, and
Direct Connectors
Method 5
6% or 8% emax
Table 4-6 or Table 4-7
Urban
Frontage Roads
Method 2
Table 4-4
Method 5
4%, 6% or 8% emax
Table 4-5, Table 4-6 or Table 4-7
Method 5
6% or 8% emax
Table 4-6 or Table 4-7
Rural Frontage Roads
Method 5
6% or 8% emax
Table 4 6 or Table 4 7
Urban Ramps for Grade
Separations on Non-Access
Controlled Facilities
Method 2
Table 4 4
Method 5
4%, 6% or 8% emax
Table 4-5, Table 4-6 or Table 4-7
Method 5
6% or 8% emax
Table 4-6 or Table 4-7
Rural Ramps for Grade
Separations on Non-Access
Controlled Facilities
Method 5
6% or 8% emax
Table 4 6 or Table 4 7
Roundabouts and Alternative
Intersections
(Including Approaches)
2
Method 2
Table 4-4
Method 5
4%, 6% or 8% emax
Table 4-5, Table 4-6 or Table 4-7
N/A
Temporary Traffic Control
3
Method 2 expanded
Table 23-1
Low-Volume Off-System Bridges
(approach roadway)
Meet or improve conditions that are typical on the remainder of the roadway
Notes:
  1. The designer has the option of selecting a superelevation methodology that either increases the radius and or decreases the side force for driver comfort on the various facility types listed in this table.
  2. The desired target speed approaching and through roundabouts and other alternative intersection forms should be lower to ensure the proper functioning of the alternative intersection and the resultant safety benefits.
  3. For areas in the TCP phasing of construction that represent what will be left in place for the final permanent condition, the respective permanent condition superelevation method should be utilized.
Table 4-4: Superelevation Rates on Low-speed Rural Town, Suburban, Urban and Urban Core Facilities (Method 2)
Design Speed
e
(%)
15
mph
R (ft)
20
mph
R (ft)
25
mph
R (ft)
30
mph
R (ft)
35
mph
R (ft)
40
mph
R (ft)
45
mph
R (ft)
-4.0
2
54
116
219
375
583
889
1227
-3.0
2
52
111
208
353
544
821
1125
-2.8
2
51
110
206
349
537
808
1107
-2.6
2
51
109
204
345
530
796
1089
-2.5
2,3
51
109
203
343
527
790
1080
-2.4
2
51
108
202
341
524
784
1071
-2.2
2
50
108
200
337
517
773
1055
-2.0
50
107
198
333
510
762
1039
-1.5
4,5
49
105
194
324
495
736
1000
-1.0
4,5
48
103
189
316
480
711
964
-0.5
4,5
48
101
185
308
467
688
931
0
5,6
47
99
181
300
454
667
900
0.5
5
46
97
177
293
441
646
871
1.0
5
45
95
174
286
430
627
844
1.5
5
45
94
170
279
419
610
818
2.0
44
92
167
273
408
593
794
2.2
44
91
165
270
404
586
785
2.4
44
91
164
268
400
580
776
2.6
43
90
163
265
396
573
767
2.8
43
89
161
263
393
567
758
3.0
43
89
160
261
389
561
750
3.2
43
88
159
259
385
556
742
3.4
42
88
158
256
382
550
734
3.6
42
87
157
254
378
544
726
3.8
42
87
155
252
375
539
718
4.0
42
86
154
250
371
533
711
Notes:
  1. Computed using Superelevation Distribution Method 2. See AASHTO’s A Policy on Geometric Design of Highways and Streets for the different types of Superelevation Distribution Methods.
  2. Normal crown values beyond -2.0% should be used for surfaces such as gravel, crushed stone, and earth.
  3. Areas with paved surfaces that receive more frequent rainfall events with high intensities and greater depths than other areas may use 2.5% normal crown.
  4. For the purpose of evaluating existing conditions, normal crown values up to -1.5% may be used
  5. Values ranging from -1.5% to +1.5% should only be used in special circumstances such as intersections
  6. 0% is provided for information purposes only and should not be used for design.
Table 4-4
, which is based on the Method 2 superelevation distribution, shows the relationship of radius, superelevation rate, and design speed for low-speed rural town, suburban, urban, and urban core facility design and should be used to evaluate existing conditions or the need for superelevation for proposed conditions on low-speed rural town, suburban, urban, and urban core facilities.
This table may also be used for design of detour alignments in constrained conditions.
For a normal crown section, the negative e-value (the slope on the outside of the curve) will always be the controlling value for a given design speed.
Example: Given a design speed of 35 mph and a 400-ft radius curve,
Table 4-4
indicates an approximate superelevation rate of 2.4 percent should be used.