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Section 4: Horizontal Alignment

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Overview

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.

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General 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.
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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.

Anchor: #BGBJCCFITable 2-3: Horizontal Curvature of High-Speed Highways and Connecting Roadways with Superelevation

(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.



Anchor: #i1063544Table 2-3: (continued): Horizontal Curvature of High-Speed Highways and Connecting Roadways with Superelevation

(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.



Anchor: #BGBFHEDGTable 2-4: Horizontal Curvature of Highways without Superelevation1

(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.

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Superelevation

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