230.1 horizontal alignment

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Green Book Figure 3-22b "Design Controls for Stopping Sight Distance on Horizontal Curves"
Green Book Figure 3-23 "Diagram Illustrating Components for Horizontal Sight Distance"
Standard Plan 203.20
Standard Plan 203.21

The operational characteristics of a roadway are directly affected by the horizontal alignment. The designer must consider the roads terrain, traffic volume, expected capacity and LOS together with other safety factors in order to properly anticipate the posted speed. Highways will be designed according to their anticipated posted speed as opposed to an arbitrary design speed.

The topography of the land traversed has an influence on the alignment of roads and streets. Variations in topography are generally separated into three classifications according to terrain:

  • Level---Highway sight distances, as governed by horizontal and vertical restrictions, are generally long or can be made so without construction difficulty of major expense.
  • Rolling---Natural slopes consistently rise above and fall below the road or street grade, and occasional steep slopes offer some restriction to the normal horizontal and vertical alignment.
  • Mountainous---Longitudinal and transverse changes in the elevation of the ground with respect to the road or street are abrupt, with benching and side excavation frequently needed to obtain acceptable horizontal and vertical alignment.

These classifications pertain to the general character of a specific roadway corridor. Roadways in valleys, passes, or mountainous areas that have all the characteristics of roads or streets traversing level or rolling terrain should be classified as level or rolling.

Additional Information for Horizontal Curvature
The basic design criteria for horizontal curvature are based upon the information contained in Chapter 3 of the AASHTO publication A Policy on Geometric Design of Highways and Streets.

Once the anticipated posted speed is identified, the road’s horizontal alignment can be selected. All horizontal curve data is defined by radius. The basic design criteria for horizontal curvature are based upon the information contained in Chapter 3 of the AASHTO publication A Policy on Geometric Design of Highways and Streets (the Green Book). Additional guidance contained in the Green Book will also be used to determine other horizontal alignments.

Terrain, traffic volume and the anticipated posted speed must be considered when establishing a roadway’s minimum horizontal curvature.

A sharper curve may be designed for an improvement if it is properly signed and superelevated. Proper signage is required to inform the driver of this condition.

Horizontal curves are used when tangent lines of the roadway intersect at an angle exceeding ten minutes (for roadways with fewer than 400 vehicles per day, curves are not required for intersection angles of 1 degree or less). The centerline of median is generally used as the survey base line for divided roadway improvements. When the median width exceeds 100 ft. or individual roadways are not parallel, consideration should be given to using the inside edge of each travelway for the survey baseline for each individual alignment. For undivided pavements, the survey base line is at the center of the travelway pavement. Existing centerlines are used where practicable.

230.1.1 English Definition

Instead of degree of curve, all horizontal curve data is described by radius. The arc definition of curvature is used for new alignment. The chord definition is used only when an existing chord defined curve is utilized in the alignment. Identify a curve that is based on the chord definition by placing "(chord)" after the radius listed in the curve data on the plans.

230.1.2 Metric Definitions

When an existing curve, either arc or chord definition, is utilized in a metric alignment it will be soft converted using the U.S. Survey Foot (U.S. survey foot factor is 0.304800609601219 meters per foot.).

230.1.3 Length of Curve

For small deflection angles, curves will be sufficiently long to avoid the appearance of a kink. Curves will be at least 500 ft. in length for a central angle of 5° and increase 100 ft. for each 1° decrease in the central angle. For anticipated posted speeds of 60 mph or less, the minimum length of a curve is about 15 times the anticipated posted speed (mph).

For anticipated posted speeds greater than 60 mph, controlled access facilities that use flat curvature, a desirable length of curve is about 30 times the anticipated posted speed (mph).

230.1.4 Superelevation and Widening

Superelevation rate and widening for various design categories are given on Standard Plan 203.20 and Standard Plan 203.21, while the length of transitions are provided in Standard Plan 203.22. For divided pavements with narrow medians of 16 ft. or less in width, the superelevation runoff length for undivided roadways is used. For multiple lane roadways, the superelevation runoff length is multiplied by the appropriate factor based on the number of lanes. In urban areas where roadways with more than four lanes intersect at grade, consideration is given to reducing the pavement cross slope to minimize the difference in elevation between the extreme edges of the travelways. The difference in the rate of cross slopes for adjacent lanes should not exceed 5%, or 0.05 ft./ft. When the plans utilize the data given on the standard plans, only the maximum rate of superelevation and the maximum widening are shown with the horizontal curve data on the plans. Complete details of the superelevation and widening transition must be given on the plans if the standard plans details are not used.

230.1.5 Spiral Transition Curves

Spiral curves are used on all roadways that have design traffic greater than 400 vehicles per day and have a radius less than the values listed on Standard Plan 203.20. The length of spiral is the same as the length of superelevation and widening transition given in Standard Plan 203.22.

230.1.6 Sight Distance

Figures 3-22b and 3-23 of the AASHTO Green Book (a graph diagram and equations) are used to determine the curve radius required to provide necessary horizontal sight distance for stopping and/or passing on roadway improvement projects.

230.1.7 Bridge Considerations

It is desirable to avoid vertical and horizontal curves on bridges. When a horizontal curve is unavoidable on a bridge, spirals and minimum horizontal clearance will be maintained between the roadway and the bridge separation structure.

230.1.8 Curve Data on the Plans

Example plans are available to illustrate how required curve data are to be shown.

230.1.9 Stationing for the Main Roadway

Stationing for roadway improvement projects proceed from north to south or from west to east.

If the project is an improvement of an existing route, the stationing of the existing route is used. The ends of the proposed improvement are tied to the existing route and equated when applicable.

230.1.10 Stationing for Crossroads

Stationing of intersected roadway is based on the existing stationing, if it exists. For crossroads where there is no existing stationing, the stationing proceeds from the left side of the intersection to the right side. The stationing is chosen so a five-station increment occurs at the intersection with the main roadway (ie. 15+00.00).

A sufficient distance is allowed on the left side of an interchange to provide the proposed improvement without having to use negative stationing. The crossroad stationing is equated to the main roadway stationing at the point of intersection and the angle of intersection is indicated.

230.1.11 Ramp Base Lines and Stationing

The base line for a ramp is located along the right edge of the ramp travelway relative to the direction of traffic. The stationing is carried in the direction of traffic except for diamond interchange ramps. Diamond interchange ramps are stationed in the same direction as the main roadway stationing. Ramp base lines are equated to the main roadway or cross road at their termini.