Geometric highway design pertains to the visible features of the highway. It may be considered as the tailoring of the highway to the terrain, to the controls of the land usage, and to the type of traffic anticipated.
Design parameters covering highway types, design vehicles, and traffic data are included in Section 2 "General Design Criteria."
This section covers design criteria and guidelines on the geometric design elements that must be considered in the location and the design of the various types of highways. Included are criteria and guidelines on sight distances, horizontal and vertical alignment, and other features common to the several types of roadways and highways.
In applying these criteria and guidelines, it is important to follow the basic principle that consistency in design standards is of major importance on any section of road. The highway should offer no surprises to the driver in terms of geometrics. Problem locations are generally at the point where minimum design standards are introduced on a section of highway where otherwise higher standards should have been applied. The ideal highway design is one with uniformly high standards applied consistently along a section of highway, particularly on major highways designed to serve large volumes of traffic at high operating speeds.
Stopping sight distance also is to be provided for all elements of interchanges and intersections at grade, including driveways.
Sight Distances for Design
4.2.2 Passing Sight Distance
Passing sight distance is considered only on two-lane roads. At critical locations, a stretch of four-lane construction with stopping sight distance is sometimes more economical than two lanes with passing sight distance.
4.2.3 Stopping Sight Distance
The stopping sight distances shown in Table 4-1 should be increased when sustained downgrades are steeper than 3 percent. Increases in the stopping sight distances on downgrades are indicated in A Policy on Geometric Design of Highways and Streets, AASHTO, 2001.
4.2.4 Stopping Sight Distance on Vertical Curves
4.2.5 Stopping Sight Distance on Horizontal Curves
Stopping sight distance for passenger vehicles on horizontal curves is obtained from Figure 4-A. For sight distance calculations, the driver's eyes are 3.5 feet above the center of the inside lane (inside with respect to curve) and the object is 2 feet high. The line of sight is assumed to intercept the view obstruction at the midpoint of the sight line and 2.75 feet above the center of the inside lane. Of course, the midpoint elevation will be higher or lower than 2.75 feet, if it is located on a sag or crest vertical curve respectively. The clear distance (M) is measured from the center of the inside lane to the obstruction.
The general problem is to determine the clear distance from the centerline of inside lane to a median barrier, retaining wall, bridge pier, abutment, cut slope, or other obstruction for a given design speed. Using radius of curvature and sight distance for the design speed, Figure 4-A gives the middle ordinate (M) which is the clear distance from centerline of inside lane to the obstruction. When the design speed and the clear distance to a fixed obstruction are known, this figure also gives the required minimum radius which satisfies these conditions.
When the required stopping sight distance would not be available because of an obstruction such as a railing or a longitudinal barrier, the following alternatives shall be considered: increase the offset to the obstruction, increase the horizontal radius, or do a combination of both. However, any alternative selected should not require the width of the shoulder on the inside of the curve to exceed 12 feet, because the potential exists that motorists will use the shoulder in excess of that width as a passing or travel lane.
When determining the required middle ordinate (M) distance on ramps, the location of the driver's eye is assumed to be positioned 6 feet from the inside edge of pavement on horizontal curves.
The designer is cautioned in using the values from Figure 4-A since the stopping sight distances and middle ordinates are based upon passenger vehicles. The average driver's eye height in large trucks is approximately 120 percent higher than a driver's eye height in a passenger vehicle. However, the required minimum stopping sight distance can be as much as 50 percent greater than the distance required for passenger vehicles. On routes with high percentages (10 percent or more) of truck traffic, the designer should consider providing greater horizontal clearances to vertical sight obstructions to accommodate the greater stopping distances required by large trucks. The approximate middle ordinate (M) required for trucks is 2.5 times the value obtained from Figure 4-A for passenger vehicles.
In designing the roadway to provide a particular stopping sight distance the designer is advised to consider alternatives. A wider sidewalk, shoulder or bike lane increases the sight triangle, see Section 6.3. Curb extensions and parking restrictions allow the driver to see pedestrians and cross traffic more easily.
A Roadway Design Tool is also available to calculate the Radius of a Horizontal Curve with a Sight Obstruction.
The major considerations in horizontal alignment design are: safety, grade, type of facility, design speed, topography and construction cost. In design, safety is always considered, either directly or indirectly. Topography controls both curve radius and design speed to a large extent. The design speed, in turn, controls sight distance, but sight distance must be considered concurrently with topography because it often demands a larger radius than the design speed. All these factors must be balanced to produce an alignment that is safe, economical, in harmony with the natural contour of the land and, at the same time, adequate for the design classification of the roadway or highway.
When the standard superelevation for a horizontal curve cannot be met, a design exception will be required. However, the highest practical superelevation should be selected for the horizontal curve design.
A 6 percent maximum superelevation rate shall be used on rural highways and rural or urban freeways (see Figure 4-B). A 4 percent maximum superelevation rate may be used on high speed urban highways to minimize conflicts with adjacent development and intersecting streets (see Figure 4-C). Low speed urban streets can use a 4 percent (See Figure 4-C) or 6 percent maximum superelevation rate (see Figure 4-C1)
A Roadway Design Tool is also available to calculate the Safe Speed for Horizontal Curves With V Greater Than 50 MPH and the Safe Speed for Horizontal Curves With V Less Than or Equal to 50 MPH.
A. Axis of Rotation
1. Undivided Highways
Where the initial median width is 30 feet or less, the axis of rotation should be at the median centerline.
(b.) Other Divided Highways
The axis of rotation should be considered on an individual project basis and the most appropriate case for the conditions should be selected.
B. Superelevation Transition
C. Transition Curves and Superelevation
B. Curve Radii for Horizontal Curves
Desirable Tangent Length Between
|Design Speed(mph)||Desirable Tangent (ft)|
|500 - 600
600 - 800
800 - 1000
H. Broken Back Curves
A broken back curve consists of two curves in the same direction joined by a short tangent. Broken back curves are unsightly and violate driver expectancy. A reasonable additional expenditure may be warranted to avoid such curvature.
The intervening tangent distance between broken back curves should, as a minimum, be sufficient to accommodate the superelevation transition as specified in Section 4.3.2. For design speeds of 50 mph and greater, longer tangent lengths are desirable. Table 4-7 indicates the desirable tangent length between same direction curves. The desirable tangent distance should be exceeded when both curves are visible for some distance ahead.
|Design Speed (mph)||Desirable Tangent (ft)|
I. Alignment at Bridges
Superelevation transitions on bridges almost always result in an unsightly appearance of the bridge and the bridge railing. Therefore, if at all possible, horizontal curves should begin and end a sufficient distance from the bridge so that no part of the superelevation transition extends onto the bridge. Alignment and safety considerations, however, are paramount and shall not be sacrificed to meet the above criteria.
The profile line is a reference line by which the elevation of the pavement and other features of the highway are established. It is controlled mainly by topography, type of highway, horizontal alignment, safety, sight distance, construction costs, cultural development, drainage and pleasing appearance. The performance of heavy vehicles on a grade must also be considered. All portions of the profile line must meet sight distance requirements for the design speed of the road.
In flat terrain, the elevation of the profile line is often controlled by drainage considerations. In rolling terrain, some undulation in the profile line is often advantageous, both from the standpoint of truck operation and construction economy. But, this should be done with appearance in mind; for example, a profile on tangent alignment exhibiting a series of humps visible for some distance ahead should be avoided whenever possible. In rolling terrain, however, the profile usually is closely dependent upon physical controls.
In considering alternative profiles, economic comparisons should be made. For further details, see the AASHTO publication: A Policy on Geometric Design of Highways and Streets, 2001.
4.4.2 Position with Respect to Cross Section
The profile line should generally coincide with the axis of rotation for superelevation; Its relation to the cross section should be as follows.
The profile line should coincide with the highway centerline.
Ramps and Freeway to Freeway Connections
The profile line may be positioned at either edge of pavement, or centerline of ramp if multi-lane.
The profile line may be positioned at either the centerline of the median or at the median edge of pavement. The former case is appropriate for paved medians 30 feet wide or less. The latter case is appropriate when:
The median edges of pavement of the two roadways are at equal elevation.
The two roadways are at different elevations.
4.4.3 Separate Grade Lines
Separate or independent profile lines are appropriate in some cases for freeways and divided arterial highways.
They are not normally considered appropriate where medians are less than 30 feet. Exceptions to this may be minor differences between opposing grade lines in special situations.
In addition, appreciable grade differentials between roadbeds should be avoided in the vicinity of at-grade intersections. For traffic entering from the crossroad, confusion and wrong-way movements could result if the pavement of the far roadway is obscured due to an excessive differential.
4.4.4 Standards for Grade
The minimum grade rate for freeways and land service highways with a curbed or bermed section is 0.3 percent. On highways with an umbrella section, grades flatter than 0.3 percent may be used where the shoulder width is 8 feet or greater and the shoulder cross slope is 4 percent or greater.
For maximum grades for urban and rural land service highways and freeways, see Table 4-8.
|Rural Land Service Highways|
|Design Speed (mph)|
|Urban Land Service Highways|
|Design Speed (mph)|
|Design Speed (mph)|
* Grades one percent steeper than the value shown for freeways in Table 4-8 may be used for
extreme cases in urban areas where development precludes the use of flatter grades for one-way
downgrades except in mountainous terrain.
4.4.5 Vertical Curves
Properly designed vertical curves should provide adequate sight distance, safety, comfortable driving, good drainage, and pleasing appearance. On new alignments or major reconstruction projects on existing highways, the designer should, where practical, provide the desirable vertical curve lengths. The use of minimum vertical curve lengths should be limited to existing highways and those locations where the desirable values or values greater than the minimum would involve significant social, environmental or economic impacts.
A parabolic vertical curve is used to provide a smooth transition between different tangent grades. Figures 4-I and 4-J give the length of crest and sag vertical curves for various design speeds and algebraic differences in grade. The stopping sight distance for these curves are based upon a height of eye of 3.5 feet, and a height of object of 2 feet. The minimum desirable length of vertical curve may also be obtained by multiplying the K value Figures 4-I or 4-J by the algebraic difference in grade. The vertical lines in Figures 4-I and 4-J are equivalent to 3 times the design speed. To determine the length of crest vertical curves on highways designed with two-way left-turn lanes, see Section 6.7.1.
Roadway Design Tools are available to calculate the
Sight Distance on a Crest Vertical Curve when the Sight Distance is Greater than the Length of Curve
Sight Distance on a Crest Vertical Curve when the Sight Distance is Less than the Length of Curve
Sight Distance on a Sag Vertical Curve when the Sight Distance is Greater than the Length of Curve
Sight Distance on a Sag Vertical Curve when the Sight Distance is Less than the Length of Curve
Flat vertical curves may develop poor drainage at level sections. Highway drainage must be given more careful consideration when the design speed exceeds 60 and 65 mph for crest vertical curves and sag vertical curves respectively. The length of sag vertical curves for riding comfort should desirably be approximately equal to:
L = AV2/46.5
L = Length of sag vertical curve, feet
A = Algebraic difference in grades, percent
V = Design speed, mph
When the difference between the P.V.I. elevation and the vertical curve elevation at the P.V.I. station (E) is 0.0625 feet (3/4 inch), a vertical curve is not required. The use of a profile angle point is permitted.
The maximum algebraic difference in tangent grades (A) that an angle point is permitted for various design speeds is shown in Table 4-9. This table is based on a length of vertical curve of 3 times the design speed.
|Design Speed (mph)||AMAX (percent)|
All umbrella section low points in cut and fill sections on freeways and Interstate highways shall be provided with slope protection at each low point in the mainline or ramp vertical geometry as shown in the Standard Roadway Construction Details. The purpose of this treatment is to minimize maintenance requirements in addressing the gradual build up of a berm which may eventually contribute to water ponding on the roadway surface and/or erosion of the side slope. The following are some recommended low point treatments:
Low Point at Edge of Ramp or Outside Edge of Mainline Pavement
Where practical, an "E" inlet should be provided in the outside edge of pavement at the low point to catch and divert the surface runoff. Provide outlet protection where needed at the pipe outfall.
As an alternative, provide slope protection which shall consist of the following:
Slope protection shall consist of a 20 foot long bituminous concrete paved area between the edge of pavement and the hinge point (PVI) together with a riprap stone flume on the fill slope and a riprap stone apron at the bottom of the slope. The riprap shall only be provided where the fill slope is steeper than 4H:1V. Where there is an inlet in a swale at the low point, center the riprap stone apron around the inlet. Where guide rail exists at the low point, the 10 foot long paved area shall be constructed instead of the non-vegetative surface treatment under the guide rail.
Slope protection shall consist of a 20 foot long bituminous concrete paved area between the edge of pavement and the toe of slope.
Low Point at Median Edge of Mainline Pavement
Provide slope protection which shall consist of a 20 foot long by 5 foot wide strip of bituminous concrete pavement adjacent to the inside edge of shoulder. If the fill slope is steeper than 4H:1V, provide riprap stone slope protection as described in "Low Point at Edge of Ramp or Outside Edge of Mainline Pavement".
On two-lane roads, extremely long crest vertical curves over one half mile should be avoided, since many drivers refuse to pass on such curves, despite adequate sight distance. It is sometimes more economical to use four-lane construction, than to obtain passing sight distance by the use of a long vertical curve.
Vertical curves affect intersection sight distance, therefore, utilizing the distances in Figures 6-A , an eye height of 3.5 feet and an object height of 3.5 feet; check for vertical sight distance at the intersection.
Broken back vertical curves consist of two vertical curves in the same direction, separated by a short grade tangent. A profile with such curvature normally should be avoided.
4.4.6 Heavy Grades
Except on level terrain, often it is not economically feasible to design a profile that will allow uniform operating speeds for all classes of vehicles. Sometimes, a long sustained gradient is unavoidable.
From a truck operation standpoint, a profile with sections of maximum gradient broken by length of flatter grade is preferable to a long sustained grade only slightly below the maximum allowable. It is considered good practice to use the steeper rates at the bottom of the grade, thus developing slack for lighter gradient at the top or elsewhere on the grade.
4.4.7 Coordination with Horizontal Alignment
A proper balance between curvature and grades should be sought. When possible, vertical curves should be superimposed on horizontal curves. This reduces the number of sight distance restrictions on the project, makes changes in profile less apparent, particularly in rolling terrain, and results in a pleasing appearance. For safety reasons, the horizontal curve should lead the vertical curve. On the other hand, where the change in horizontal alignment at a grade summit is slight, it safely may be concealed by making the vertical curve overlay the horizontal curve.
When vertical and horizontal curves are thus superimposed, the superelevation may cause distortion in the outer pavement edges. Profiles of the pavement edge should be plotted and smooth curves introduced to remove any irregularities.
A sharp horizontal curve should not be introduced at or near a pronounced summit or grade sag. This presents a distorted appearance and is particularly hazardous at night.
A climbing lane, as shown in Figure 4-K, is an auxiliary lane introduced at the beginning of a sustained positive grade for the diversion of slow traffic.
Generally, climbing lanes will be provided when the following conditions are satisfied. These conditions could be waived if slower moving truck traffic was the major contributing factor causing a high accident rate and could be corrected by addition of a climbing lane.
The following three conditions should be satisfied to justify a climbing lane:
a. Upgrade traffic flow rate in excess of 200 vehicles per hour.
b. Upgrade truck flow rate in excess of 20 vehicles per hour.
c. One of the following conditions exists:
(1) A 10 mph or greater speed reduction is expected for a typical heavy truck.
(2) Level of Service E or F exists on the grade.
(3) A reduction of two or more levels of service is experienced when moving from the approach segment of the grade.
A complete explanation and a sample calculation on how to check for these conditions are shown in the section on "Climbing Lanes" contained in "Chapter III, Elements of Design", A Policy on Geometric Design of Highways and Streets, AASHTO, 2001.
Freeways and Multi-lane Highways
Both of the following conditions should be satisfied to justify a climbing lane:
a. A 10 mph or greater speed reduction is expected for a typical heavy truck.
b. The service volume on an individual grade should not exceed that attained by using the next poorer level of service from that used for the basic design. The one exception is that the service volume derived from employing Level of Service D should not be exceeded.
The beginning warrant for a truck climbing lane shall be that point where truck operating speed is reduced by 10 mph. To locate this point, use Exhibit 3-59 or Exhibit 3-63 of the aforementioned AASHTO manual, depending on the weight/horsepower ratio of the appropriate truck. The beginning of the climbing lane should be preceded by a tapered section, desirably 300 feet, however, a 150 foot minimum taper may be used.
Desirably, the point of ending of a climbing lane would be to a point beyond the crest, where a typical truck could attain a speed that is about 10 mph below the operating speed of the highway. This point can be determined from Exhibit 3-60 of the aforementioned AASHTO manual. If this is not practical, end the climbing lane at a point where the truck has proper sight distance to safely merge into the normal lane, or preferably, 200 feet beyond this point. For two lane highways, passing sight distance should be available. For freeways and multi-lane highways, passing sight distance need not be considered. For all highways, as a minimum, stopping sight distance shall be available. The ending taper beyond this point shall be according to Figure 4-L.
A distance-speed profile should be developed for the area of a climbing lane. The profile should start at the bottom of the first long downgrade prior to the upgrade being considered for a climbing lane, speeds through long vertical curves can be approximated by considering 25 percent of the vertical curve length (chord) as part of the grade under question.
Design standards of the various features of the transition between roadways of different widths should be consistent with the design standards of the superior roadway. The transition for a lane drop or lane width reduction should be made on a tangent section whenever possible and should avoid locations with horizontal and vertical sight distance restrictions. Whenever feasible, the entire transition should be visible to the driver of a vehicle approaching the narrower section.
The design should be such that at-grade intersections within the transition are avoided.
Figure 4-L shows the minimum required taper length based upon the design speed of the roadway. In all cases, a taper length longer than the minimum should be provided where possible. In general, when a lane is dropped by tapering, the transition should be on the right so that traffic merges to the left.
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