ARTICLE IV

Design Standards

4.3   Geometric Standards

4.3.1        Horizontal Alignment

4.3.1.1  General Considerations
The major considerations in horizontal alignment are: topography, road classification, design speed, grade profile, subsurface conditions, safety, sight distance and construction costs. All of these shall be balanced to produce an alignment that is safest, most economical and adequate for the functional classification of the road.

4.3.1.2  Types of Horizontal Curves
Figure 4-1 shows simple and spiral curves and their functions.

4.3.1.3  Stopping Sight Distance
Horizontal alignment shall provide at least minimum stopping sight distance for the design speed at all points. This includes visibility at intersections as well as around curves and roadside appurtenances. The minimum stopping sight distance is the distance required by the driver of the vehicle traveling at a given speed to bring the vehicle to a stop after an object on the road becomes visible. Stopping sight distance is measured from the driver’s eye that is assumed to be 3.50 feet above the roadway surface, to an object six inches high on the road. The required stopping distance for a given design speed is as follows:

Design Speed

(MPH)

Stopping Sight

Distance (ft.)

15

20

25

30

35

40

50

60

100

150

175

200

250

300

450

650

In addition to stopping sight distance, intersection sight distance must be provided where applicable. Section 4.7.5 provides the standards for Intersection sight distance. In some cases passing sight distance may be required on collectors or Arterials. Passing sight distance is given in the Colorado Department of Transportation Roadway Design Manual. Where an object off the pavement such as a bridge pier, cut slope, or natural growth, restricts sight distance, the stopping sight distance determines the minimum radius curvature. Offset clearance to achieve stopping sight distance on horizontal curves is shown in Figure 4-2. It is assumed that the driver’s eye and the object are centered in the inside lane, and the line of sight is assumed to intercept the obstruction at the mid-point of the sight line and 2.5 feet above the inside lane. The offset distance (m) is measured from the center of the inside lane to the obstruction. FIGURE 4-1 FIGURE 4-2

4.3.1.4  Standards for Curvature 
Tables 4-1 through 4-4 give the minimum curve radii and the maximum allowable rate of superelevation for the various functional classifications. The tables are based on design speed, friction factors and superelevation and do not consider sight distance. Minimum radii should be used only when the cost of realizing the higher standard is inconsistent with the benefit. Sudden reductions in standards introduce the element of surprise to the driver and should be avoided. Where physical restrictions cannot be overcome and it becomes necessary to introduce curvature or a lower standard than the design speed for the project, the design speed between successive curves shall not change by more than 10 mile-per-hour increments. Under no conditions shall a curve for a design speed lower than the design speed of the project be introduced at the end of a long tangent or at other locations where high approach speeds may be anticipated. Angle points less than one degree require no curve radius. A compound curve will not be permitted. A broken-back curve is two curves in the same direction joined by a short tangent. Broken-back curves are not permitted.

4.3.1.5  Superelevation 
One of the most important factors to consider in highway safety is the centrifugal force generated when a vehicle traverses a curve. Centrifugal force increases as the velocity of the vehicle and/or degree of curvature increases. The standard superelevation rates shown on Table 4-1 through 4-4 are such as to hold the side friction factor within tolerable limits for those operating speeds expected for the range of curve radii given. For undivided roads, the axis of rotation for superelevation is usually the centerline. Where long relatively level tangents precede curves, however, the plane of superelevation may be rotated about the edge of pavement to improve perception of the curve. Drainage pockets are caused when the axis of rotation is from the centerline instead of the inside edge of pavement. A superelevation transition is variable in length depending upon the amount of superelevation. With respect to the beginning and end of the curve, two- thirds of the transition is in the tangent approach and one-third of the full superelevation at the beginning and at the end of the curve. Where spiral curves are permitted, the transitions are to be designed using Colorado Department of Transportation Roadway Design Manual. After a superelevation transition is computed, profiles of the pavement edges should be plotted and irregularities removed by introducing smooth curves. For wide pavements, it is often advantageous to plot intermediate profiles. On curved roadways, pronounced sag may develop on the low side of the superelevation. This is corrected by adjusting the grades on two edges of pavement throughout the curve.

4.3.1.6  Alignment at Bridges 
Ending a curve on a bridge is undesirable and adds to the complication of design and construction. Likewise, curves beginning or ending near a bridge should be placed so that no parts of the spiral or superelevation transitions extend onto the bridge. If curvature is unavoidable, every effort should be made to keep the bridge within the limits of a simple curve.

Table 4 -1 MINIMUM CURVE RADIUS FOR DESIGN SPEED

ON URBAN RESIDENTIAL STREETS (without superelevation)

e - superelevation

f - side friction factor

V - design speed

R - curve radius

v

15

20

25

30

e

0

0

0

0

f

.19

.18

.17

.16

Radius

80

150

250

375

 

Table 4 -2 MINIMUM CURVE RADIUS FOR DESIGN SPEED ON

COLLECTORS, MINOR ARTERIALS AND PRINCIPAL ARTERIALS

Design Speed

v

e

Max f

Min R

e

Max f

Min R

e

Max f

Min R

40

.04

.15

561

.06

.15

508

.08

.15

464

50

.04

.14

926

.06

.14

833

.08

.14

758

60

.04

.13

1412

.06

.13

1263

.08

.13

1143

 

 

Table 4 - 3 MAXIMUM SUPERELEVATION RATES

Road Type

Rural

Urban

Principal Arterial

Minor Arterial

Collector

Residential Sub Collector

.08

.08

.06

.06

.06

.06

.06

.04

 

Table 4 - 4 SIDE FRICTION FACTORS FOR DESIGN SPEED

v

f

15

20

25

30

40

60

.19

.18

.17

.16

.15

.13

 

 

4.3.1.7  Coordination with Vertical Alignment
To avoid the possibility of introducing serious traffic hazards, coordination is required between horizontal and vertical alignment. Particular care shall be exercised to maintain proper sight distance at all times. Sharp horizontal curves introduced at or near the top of pronounced crest or bottom of sag vertical curves should be avoided. Wherever possible, vertical curves should be superimposed on horizontal curves. This reduces the number of sight distance restrictions on a given length of road and makes changes in profile less apparent, particularly in flat or rolling terrain. For safety reasons, the horizontal curve should overlap the vertical curve. Where the change in horizontal alignment at a grade summit is slight, however, the vertical curve may overlap the horizontal. When vertical and horizontal curves are superimposed, the resulting superelevation may cause distortion in the outer pavement edges, particularly on multi-lane cross- sections. Where this may be the case, edge of pavement profiles should be plotted and smooth vertical curves introduced to remove any irregularities.

4.3.2        Vertical Alignment

4.3.2.1  General Considerations
The centerline profile is a reference line by which the elevation or grades of the pavement and other features of the roadway are established. It is controlled mainly by topography structure clearances, horizontal alignment, safety, sight distance, design speed, construction costs and the performance of heavy vehicles on a grade. The centerline profile should be positioned with relation to the cross-section as follows:

*  It should coincide with the road centerline on two-lane and multi-lane undivided roads.

*  On multi-lane divided roads, the grade lines should be placed at the edge of the travel lane nearest the median.

4.3.2.2  Minimum and Maximum Grades
Minimum sustained grades shall be no less than 0.5 percent, on Urban Roads and 1.0 percent on Rural Roads. Maximum sustained grades for new roads are related to design speed as follows:

MAXIMUM SUSTAINED GRADES (%)

Terrain Classification (mph)

15

20

25

30

40

50

60

Flat and Rolling

6

6

6

6

6

5

4

Mountainous

12

10

9

9

8

6

NA

The maximum design grade should be used infrequently rather than as a value to be used in most cases. For short grades less than 200 feet, the maximum gradient may be increased by one percent. In Flat or Rolling Terrain all grades shall flatten to four percent for at least 100 feet approaching intersections and for at least 50 feet entering and leaving turnarounds or cul-de-sacs. In Mountainous Terrain all grades shall flatten to six percent or less for at least 50 feet approaching intersections and entering switchbacks or cul-de-sacs.

4.3.2.3  Vertical Curves
Properly designed vertical curves should provide adequate stopping and passing sight distance, headlight sight distance, comfortable driving, good drainage and pleasing appearance. Vertical curves shall be parabolic. Figure 4-3 gives the mathematical relations for computing a vertical curve, either at crests or sags. Design controls for vertical curves are given in Table 4-5. The minimum length vertical curve shall be 400 feet for design speeds above 30 mph and 200 feet for design speeds of 30 mph and lower. Unequal tangent vertical curves are permitted only in special circumstances as approved by the County Engineering Manager.

Table 4 -5 DESIGN CONTROLS FOR VERTICAL CURVES AT CENTERLINE

Design Speed MPH

Stopping Sight Distance

Length = “K” Times Algebraic Difference in Grades K

 

Min

Desirable

Crest

Sag

 

(ft)

(ft.)

Min

Desirable

Min

Desirable

30

40

50

60

200

275

350

475

200

300

450

650

28

55

85

160

28

65

145

300

35

55

75

105

35

60

100

155

 

Vertical curves are not required where algebraic difference is less than 0.20%. The desirable minimum length of vertical curves, both crest and sag, is 400 feet.

Vertical curves that have a level point and flat sections near their crest or sag should be evaluated for drainage where curbed pavements are used. Values of K = 143 or greater should be checked for drainage.

Also vertical curves that are long and flat may develop poor drainage at the level section. This difficulty may be overcome by adjusting the flow line of the ditch section.

4.3.2.4  Stopping Sight Distance 
Minimum lengths of crest vertical curves are controlled by stopping sight distance requirements as shown in Figure 4-4. Figure 4-3 Figure 4-4

 

4.2 Design Factors

4.4 Cross Section Standards