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Section 2: Roadway Design Criteria

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Lane Width and Number

The usual and minimum lane width is 13 ft [4 m]. The number of lanes required to accommodate the anticipated traffic in the design year is determined by the level of service evaluation as discussed in the Highway Capacity Manual.

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Shoulders

The minimum shoulder width is 12 ft [3.6 m]. This width applies to both inside and outside shoulders, regardless of the number of main lanes of the facility. Shoulders should be continuously surfaced and be maintained along all speed change lanes.

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Pavement Cross Slope

Multilane divided pavements should be inclined in the same direction. The recommended pavement cross slope is 2.0 percent. Shoulders should be sloped sufficiently to drain surface water but not to an extent that safety concerns are created for vehicular use.

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Vertical Clearances at Structures

The minimum vertical clearances at structures for these facilities are as described in Chapter 3, Section 6.

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Stopping Sight Distance

Stopping sight distance (SSD) for these facilities is calculated using the same methodology described in Chapter 2, Section 3. The key variables that affect the calculation of SSD are brake reaction time and deceleration rate.

The calculated and design stopping sight distances are shown in Table 8-1. Significant downgrades may affect stopping sight distances.

NOTE: Online users can click here to see the below table in PDF format.

Anchor: #i1017354Table 8-1: Stopping Sight Distance (US Customary)

Design Speed (mph)

Brake reaction distance (ft)

Braking distance on level (ft)

Stopping Sight Distance

-

-

-

Calculated (ft)

Design (ft)

85

312.4

693.5

1005.8

1010

90

330.8

777.5

1108.2

1110

95

349.1

866.2

1215.4

1220

100

367.5

959.8

1327.3

1330

(Metric)

Design Speed (km/h)

Braking distance on level (m)

Brake reaction distance (m)

Stopping Sight Distance

Calculated (m)

Design (m)

140

97.3

224.8

322.1

325

150

104.3

258.1

362.3

365

160

111.2

293.6

404.8

405

NOTE: brake reaction distance predicated on a time of 2.5s; deceleration rate 11.2 ft/sec[3.4 m/sec]



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Grades

Undesirable speed differentials that could occur between vehicle types on these facilities suggest that limiting the rate and length of the grades be considered. Passenger vehicles are not significantly affected by grades as steep as 3 percent, regardless of initial speed. Grades above 2 percent may affect truck traffic depending on length of grade.

Table 8-2 summarizes the maximum grade controls in terms of design speed.

NOTE: Online users can click here to see the below table in PDF format.

Anchor: #i1017436Table 8-2: Maximum Grades (US Customary)

Type of Terrain

Design Speed

-

85

90

95

100

Level

2-3

2-3

2-3

2-3

Rolling

4

4

4

4

(Metric)

-

140

150

160

--

Level

2-3

2-3

2-3

--

Rolling

4

4

4

--



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Curve Radii

The minimum curve radii for superelevation rates of 6 percent and 8 percent are shown in Table 8-3. These radii were calculated using the horizontal curvature equation shown in Chapter 2, section 4, with the side friction values in Table 8-5 and the assumed maximum superelevation rates.

NOTE: Online users can click here to see the below tables in PDF format.

Anchor: #i1017488Table 8-3: Horizontal Curvature Highways and Connecting Roadways with Superelevation (US Customary [based on emax = 6%])

Design Speed

(mph)

Usual Min. Radius of Curve (ft)

Absolute Min.1 Radius of Curve (ft)

85

5615

3710

90

6820

4500

95

8285

5470

100

10100

6670

(Metric [based on emax = 6%])

Design Speed

(km/h)

Usual Min. Radius of Curve (m)

Absolute Min.1 Radius of Curve (m)

140

1800

1190

150

2440

1615

160

3050

2020

1 Absolute minimum values should be used only where unusual design circumstances dictate.



Anchor: #i1017536Table 8-3: Horizontal Curvature Highways and Connecting Roadways with Superelevation (US Customary [based on emax = 8%])

Design Speed

(mph)

Usual Min. Radius of Curve (ft)

Absolute Min.1 Radius of Curve (ft)

85

4865

3215

90

5845

3860

95

7010

4630

100

8420

5560

(Metric [based on emax = 8%])

Design Speed

(km/h)

Usual Min. Radius of Curve (m)

Absolute Min.1 Radius of Curve (m)

140

1560

1030

150

2060

1365

160

2550

1680

1Absolute minimum values should be used only where unusual design circumstances dictate.



NOTE: Online users can click here to see the below table in PDF format.

Anchor: #i1017584Table 8-4: Side Friction Factors and Running Speeds for Horizontal Curves

(US Customary)

(Metric)

Design Speed (mph)

Side Friction Factor

Running Speed (mph)

Design Speed (km/h)

Side Friction Factor

Running Speed (km/h)

85

0.07

67

140

0.07

110

90

0.06

70

150

0.05

1181

95

0.05

751

160

0.04

1311

100

0.04

821

1Values adjusted to eliminate negative friction on curve.



Horizontal curvature without superelevation means maintaining a normal crown with a negative 2 percent superelevation for one direction, and the side friction is not excessive for that direction. Table 85 shows the minimum curve radii without additional superelevation and an emax of 8 percent.

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Anchor: #i1017637Table 8-5: Horizontal Curvature of Highways without Superelevation1 (US Customary)

Design Speed (mph)

Min. Radius (ft)

85

14700

90

16200

95

18800

100

22400

(Metric)

Design Speed (km/h)

Min. Radius (m)

140

4680

150

5480

160

6750

1Normal crown (2%) maintained (emax = 8%)



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Superelevation

The maximum superelevation rates of 6 to 8 percent are not varied based on design speed.

Tables 8-6 and 8-7 show superelevation rates (maximum 6 and 8 percent, respectively) for various design speeds and radii.

NOTE: Online users can click here to see the below table in PDF format.

Anchor: #i1017674Table 8-6: Superelevation Rates for Horizontal Curves: Superelevation Rate, e (6%), for Design Speed of (US Customary)

Radius (ft)

85 mph

90 mph

95 mph

100 mph

23000

NC

NC

NC

NC

20000

NC

NC

NC

2.2

17000

NC

NC

2.2

2.6

14000

RC

2.3

2.6

3.2

12000

2.4

2.6

3.0

3.6

10000

2.8

3.1

3.6

4.3

8000

3.4

3.8

4.5

5.3

6000

4.5

5.0

5.8

Rmin = 6670 ft

5000

5.2

5.8

Rmin = 5470 ft

-

4000

5.9

Rmin = 4500 ft

-

-

3500

Rmin = 3710 ft

-

-

-

NC = Normal Crown

RC = Reverse Crown

emax = 6%



NOTE: Online users can click here to see the below table in PDF format.

Anchor: #i1017769Table 8-6: Superelevation Rates for Horizontal Curves: Superelevation Rate, e (6%), for Design Speed of (Metric)

Radius (m)

140 km/h

150 km/h

160 km/h

7000

NC

NC

NC

5000

NC

2.1

2.6

3000

3.0

3.5

4.3

2500

3.5

4.2

5.1

2000

4.3

5.2

Rmin = 2015 m

1500

5.5

Rmin = 1610 m

-

1400

5.7

-

-

1300

5.9

-

-

1200

6.0

-

-

1000

Rmin = 1190 m

-

-

NC = Normal Crown

RC = Reverse Crown

emax = 6%



NOTE: Online users can click here to see the below table in PDF format.

Anchor: #i1017869Table 8-7: Superelevation Rates for Horizontal Curves: Superelevation Rate, e (8%), for Design Speed of (US Customary)

Radius (ft)

85 mph

90 mph

95 mph

100 mph

23000

NC

NC

NC

NC

20000

NC

NC

NC

2.2

17000

NC

NC

2.2

2.6

14000

2.1

2.3

2.7

3.2

12000

2.4

2.7

3.1

3.7

10000

2.9

3.2

3.7

4.5

8000

3.6

4.0

4.7

5.6

6000

4.8

5.3

6.2

7.4

5000

5.7

6.4

7.5

Rmin = 5560 ft

4000

7.0

7.9

Rmin = 4630 ft

-

3500

7.8

Rmin = 3860 ft

-

-

3000

Rmin = 3215 ft

-

-

-

NC = Normal Crown

RC = Reverse Crown

emax = 8%



NOTE: Online users can click here to see the below table in PDF format.

Anchor: #i1017971Table 8-7: Superelevation Rates for Horizontal Curves: Superelevation Rate, e (8%), for Design Speed of

(Metric)

Radius (m)

140 km/h

150 km/h

160 km/h

7000

NC

NC

NC

5000

NC

2.2

2.7

3000

3.1

3.6

4.5

2500

3.7

4.4

5.4

2000

4.6

5.5

6.7

1500

6.0

7.3

Rmin = 1680 m

1400

6.4

7.8

-

1300

6.9

Rmin = 1365 m

-

1200

7.4

-

-

1000

Rmin = 1030 m

-

-

NC = Normal Crown

RC = Reverse Crown

-

emax = 8%



Desirable design values for length of superelevation transition on these facilities are based on using a given maximum relative gradient between profiles of the edge of traveled way and the axis of rotation. Table 8-8 shows recommended maximum relative gradient values. Transition length on this basis is directly proportional to the total superelevation, which is the product of the lane width and the change in the cross slope.

NOTE: Online users can click here to see the below table in PDF format.

Anchor: #i1018079Table 8-8: Maximum Relative Gradient for Superelevation Transition

(US Customary)

(Metric)

Design Speed (mph)

Maximum Relative

Gradient,%1

Equivalent Maximum

Relative Slope

Design Speed (km/h)

Maximum Relative

Gradient,%1

Equivalent Maximum

Relative Slope

85

0.33

1:303

140

0.32

1:313

90

0.30

1:333

150

0.28

1:357

95

0.28

1:357

160

0.25

1:400

100

0.25

1:400

-

-

-

1Maximum relative gradient for profile between edge of traveled way and axis of rotation.



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Vertical Curves

Vertical curves create a gradual transition between different grades which is essential for the safe and efficient operation of a roadway. The lengths of both crest and sag vertical curves are controlled by the available sight distance.

K Values are calculated using the same equations as in Chapter 3, Section 4.

Design Ks for both crest and sag vertical curves are shown on Table 8-9.

NOTE: Online users can click here to see the below table in PDF format.

Anchor: #i1018132Table 8-9: Vertical Curves (US Customary)

Design Speed (mph)

Stopping Sight Distance (ft)

Crest Vertical Curves

Sag Vertical Curves

-

-

Design K

Design K

85

1010

473

260

90

1110

571

288

95

1220

690

319

100

1330

820

350

(Metric)

Design Speed (km/h)

Stopping Sight Distance (m)

Crest Vertical Curves

Sag Vertical Curves

-

-

Design K

Design K

140

325

161

84

150

365

203

96

160

405

250

107



The length of a sag vertical curve that satisfies the driver comfort criteria is 60 percent of the sag vertical curve lengths required by the sight distance control. The use of driver comfort control should be reserved for special use and where continuous lighting systems are in place.

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