Section 4: Horizontal Alignment
Anchor: #i1085865Overview
In the design of highway alignment, it is necessary to establish the proper relation between design speed and curvature. The two basic elements of horizontal curves are Curve Radius and Superelevation.
Anchor: #i1085879General Considerations for Horizontal Alignment
There are a number of general considerations which are important in attaining safe, smooth flowing, and aesthetically pleasing facilities. These practices as outlined below are particularly applicable to high-speed facilities.
- Flatter than minimum curvature for a certain design speed should be used where possible, retaining the minimum guidelines for the most critical conditions.
- Compound curves should be used with caution and should be avoided on mainlanes where conditions permit the use of flat simple curves. Where compound curves are used, the radius of the flatter curve should not be more than 50 percent greater than the radius of the sharper curve for rural and urban open highway conditions. For intersections or other turning roadways (such as loops, connections, and ramps), this percentage may be increased to 100 percent.
- Alignment consistency should be sought. Sharp curves should not follow tangents or a series of flat curves. Sharp curves should be avoided on high, long fill areas.
- Reverse curves on high-speed facilities should include an intervening tangent section of sufficient length to provide adequate superelevation transition between the curves.
- Broken-back curves (two curves in the same direction connected with a short tangent) should normally not be used. This type of curve is unexpected by drivers and is not pleasing in appearance.
- Horizontal alignment and its associated design speed should be consistent with other design features and topography. Coordination with vertical alignment is discussed in Combination of Vertical and Horizontal Alignment in Section 5.
Curve Radius
The minimum radii of curves are important control values in designing for safe operation. Design guidance for curvature is shown in Table 2-3 and Table 2-4: Horizontal Curvature of Highways without Superelevation1.
|
(US Customary [based on emax = 8%]) |
||
|---|---|---|
|
Design Speed (mph) |
Usual Min.1,2 Radius of Curve (ft) |
Absolute Min.1,3 Radius of Curve (ft) |
|
45 |
755 |
600 |
|
50 |
960 |
760 |
|
55 |
1490 |
965 |
|
60 |
1985 |
1205 |
|
65 |
2445 |
1485 |
|
70 |
3025 |
1820 |
|
75 |
3330 |
2215 |
|
80 |
4025 |
2675 |
|
(Metric [based on emax = 8%]) |
||
|
Design Speed (km/h) |
Usual Min.1,2 Radius of Curve (m) |
Absolute Min.1,3 Radius of Curve (m) |
|
70 |
220 |
175 |
|
80 |
290 |
230 |
|
90 |
470 |
305 |
|
100 |
650 |
395 |
|
110 |
830 |
500 |
|
120 |
1000 |
665 |
|
130 |
1250 |
830 |
|
1For other maximum superelevation rates refer to AASHTO’s A Policy on Geometric Design of Highways and Streets. 2 Applies to new location construction. For 3R or reconstruction, existing curvature equal to or flatter than absolute minimum values may be retained unless accident history indicates flattening curvature. 3 Absolute minimum values should be used only where unusual design circumstances dictate. |
||
|
(US Customary [based on emax = 6%]) |
||
|---|---|---|
|
Design Speed (mph) |
Usual Min.1,2 Radius of Curve (ft) |
Absolute Min.1,3 Radius of Curve (ft) |
|
45 |
830 |
660 |
|
50 |
1055 |
835 |
|
55 |
1645 |
1065 |
|
60 |
2210 |
1340 |
|
65 |
2735 |
1660 |
|
70 |
3405 |
2050 |
|
75 |
3775 |
2510 |
|
80 |
4605 |
3060 |
|
(Metric [based on emax = 6%]) |
||
|
Design Speed (km/h) |
Usual Min.1,2 Radius of Curve (m) |
Absolute Min.1,3 Radius of Curve (m) |
|
70 |
250 |
195 |
|
80 |
320 |
250 |
|
90 |
520 |
335 |
|
100 |
720 |
435 |
|
110 |
930 |
560 |
|
120 |
1140 |
755 |
|
130 |
1430 |
950 |
|
1For other maximum superelevation rates refer to AASHTO’s A Policy on Geometric Design of Highways and Streets. 2 Applies to new location construction. For 3R or reconstruction, existing curvature equal to or flatter than absolute minimum values may be retained unless accident history indicates flattening curvature. 3 Absolute minimum values should be used only where unusual design circumstances dictate. |
||
|
(US Customary) |
|
|---|---|
|
Design Speed (mph) |
Min. Radius (ft) |
|
15 |
690 |
|
20 |
1220 |
|
25 |
1760 |
|
30 |
2410 |
|
35 |
3160 |
|
40 |
4010 |
|
45 |
4970 |
|
50 |
6030 |
|
55 |
7210 |
|
60 |
8500 |
|
65 |
9590 |
|
70 |
10750 |
|
75 |
12000 |
|
80 |
13340 |
|
(Metric) |
|
|
Design Speed (km/h) |
Min. Radius (m) |
|
20 |
145 |
|
30 |
325 |
|
40 |
575 |
|
50 |
800 |
|
60 |
1100 |
|
70 |
1455 |
|
80 |
1800 |
|
90 |
2195 |
|
100 |
2685 |
|
110 |
3110 |
|
120 |
3650 |
|
130 |
4015 |
|
1 Normal crown (2%) maintained (emax = 8%) |
|
For high speed design conditions, the maximum deflection angle allowable without a horizontal curve is fifteen (15) minutes. For low speed design conditions, the maximum deflection angle allowable without a horizontal curve is thirty (30) minutes.
Anchor: #BGBIIDDDSuperelevation
As a vehicle traverses a horizontal curve, centrifugal force is counter-balanced by the vehicle weight component due to roadway superelevation and by the side friction between tires and surfacing as shown in the following equation:
e + f = V2/15R (US Customary)
Where:
e = superelevation rate, in decimal format
f = side friction factor
V = vehicle speed, mph
R = curve radius, feet
e + f = V2/127R (Metric)
Where:
e = superelevation rate, in decimal format
f = side friction factor
V = vehicle speed, km/h
R = curve radius, m
Superelevation transition is the general term denoting the change in cross slope from a normal crown section to the full superelevated section or vice versa. To meet the requirements of comfort and safety, the superelevation transition should be effected over a length adequate for the usual travel speeds. In general, the location of the transition in respect to the end of a simple (circular) curve should be such that two-thirds of the transition is outside the curve and one-third within the limits of the curve. This results in two-thirds of the full superelevation at the beginning of the curve. On curves which are spiraled, the transition usually is distributed over the length of the spiral curve. Care must be exercised in the transition, especially in curbed sections or on bridges, to avoid drainage problems and unsightly curb or bridge rail profiles.
Profiles of both gutters or pavement edges should be plotted to insure proper drainage and smoothness throughout transition sections, especially where these sections occur within vertical curvature of