"Roadway Design," Chapter 2 of Roadway Fundamentals for Municipal Officials
a workshop presented by the Maine Local Roads Center, Maine Department of Transportation, 1995

This publication is presented for "Classroom Use Only."  Its intended use is to stimulate and aid in discussion and role playing within a classroom setting.

2.5 ROAD GEOMETRY

Roads must be designed to provide safe travel for the anticipated volume at the design speed. To do this, a number of physical features of the road must be adequately provided for. In this chapter we will look at the following:

Alignment
Road width
Sight distance
Design speed
Surface friction
Superelevation

2.5.1 Alignment

The road alignment should provide a safe and comfortable ride for travel at the design speed. Unless otherwise posted, the public assumes the design speed to be 45 miles per hour. When the horizontal or vertical alignment will not allow safe travel at 45 mph, the road must be posted at the safe travel speed

Signs should be posted to warn the driver of hazards such as vertical curves that hide horizontal curves. Horizontal curves that cannot be safely negotiated at 45 mph should besigned at the safe speed. A ball-bank indicator can be mounted in a vehicle and used to determine the safe speed for horizontal curves. Horizontal curves are designed as a function of speed. The higher the design speed, the larger the curve radius must be.

The passing zones on a road are those areas where the sight distance is at least equal to the passing sight distance for the design speed. The centerline marking must be clearly painted to indicate zones of passing and no passing.

2.5.2 Road Width

Lane width is a function of road use and design speed. As a minimum, rural roads should have 18 feet of traveled way width and 2 foot shoulders (a total of 22 feet wide). For roads carrying larger volumes of vehicles and/or pedestrians, the traveled way width and shoulders need to be greater.

In Maine, a 3-rod right-of-way (49.5 ft.) is typical on local roads. This includes the road, shoulders, ditches, and any backslopes or adjacent property.

2.5.3 Sight Distance

There are two sight distances used in design:

• Stopping Sight distance
• Passing Sight distance

The stopping sight distance is the distance a vehicle travels from the instant a driver sights an object to the point at which a braking vehicle stops.

STOPPING SIGHT DISTANCE = DISTANCE FOR PERCEPTION, REACTION, and BRAKING

Figure 2-8 illustrates the distance to stop a passenger vehicle on level, wet pavement at various speeds. These distances would substantially increase if any of the following conditions existed:

- downgrade
- trucks
- bad brakes/tires
- driver impairment
- inclement weather

The passing sight distance is the distance necessary for a driver to see ahead and then overtake, pass, and return to the travel lane without interfering with another vehicle. The passing sight distance is a consideration only for two lane roads. Note that passing sight distance is considerably longer than stopping sight distance.

2.5.4 Design Speed

The design speed for a road is the speed at which the traffic is expected to travel. All geometry for the road should be designed to safely carry the design vehicle travelling at that speed. Low design speeds are applicable to roads in rough terrain; where the terrain is level, the design speed should be higher. Design speed also depends upon the road classification and the level-of-service (LOS) desired.

Type of Terrain	Current ADT Under 50	Current ADT 50-250	Current ADT 250-400	Current ADT 400 and Over
Level	30	30	40	50
Rolling	20	30	30	40
Mountainous	20	20	20	30

Figure 2-9: Minimum Design Speeds for Local Rural Roads

 2.5.5 Surface Friction

Although surface friction is not often measured, it is a characteristic that the road superintendent should consider. As surfaces wear and aggregate becomes polished, or when a surface bleeds, the road surface friction will decrease. This means longer braking distances for vehicles and more slippery conditions when the surface is wet.

2.5.6 Superelevation

With a horizontal curve, the road often has to be superelevated ("banked") in order to keep the vehicle on the road when it travels at the design speed. For a sharp curve, the superelevation is greater than it is for a gradual curve. A properly designed curve will balance the forces on a vehicle as it rounds a curve, and provide a comfortable ride. In areas where ice and snow are a factor, there are limits to the amount of superelevation that can be used. Too much bank on a curve can cause vehicles to slide to the center of the curve if the road is icy. A reasonable maximum superelevation is 1 inch per foot for asphalt surfaces, and 1 1/2 inch per foot for aggregate surfaces. The amount of superelevation on a curve cannot be built in suddenly but should "run out" at the beginning and at the end of the curve. The runout distance varies depending upon the amount of superelevation.

A number of publications are available for the road officials to use as guides in design policy and signing. Three are listed below.

Maine DOT Highway Design Guide, Maine DOT, January 1994

A Policy on Geometric Design of Highways and Streets, by the American Association of State Highway and Transportation Officials, 1994.

Manual on Uniform Traffic Control Devices, by the U. S. Department of Transportation, Federal Highway Administration, 1978.

2.6 TYPICAL SECTION

All roads need to be built and maintained to safely carry today's traffic volumes and loads. Figures 2-10 and 2-11 illustrate desirable MINIMUM widths and slopes for a low volume average paved and unpaved road respectively.

2.7 DETERMINING THE "DEGREE-OF-CURVE" IN THE FIELD

A horizontal curve in a road can be a slow, smooth curve or a sharp, angular curve. The "sharpness" of a curve can be defined by its "degree-of-curve".

As shown in the following Figure, the "degree-of-curve" is the number of degrees created by a 100 ft. long arc length.

As a curve's "sharpness" increases, the "degree-of-curve" increases. For example, a long, slow curve on I-95 may be a 1degree curve, whereas a residential, "sharp" curve may be a 20 degree curve.

To measure the "degree-of-curve", measure the distance from the curve's centerline (may not be the painted line) at it's sharpest point to the 62 foot "chord". This distance is called the "middle ordinate". Express the distance in inches and decimals of an inch The result will be the "degree-of curve". For example, if the distance is 6 1/2 inches, this is equivalent to 6.5 inches. The "degree-of curve" is 6.5 degrees. If the distance is 1 3/4 inches, then the "degree-of-curve" is 1.75 degrees.

Knowing a curve's "degree-of-curve", you can now directly calculate the "curve radius". As a curve's "sharpness" increases, the radius decreases. For example, a long, gradual curve may have a radius of over 5000 ft whereas the curve at a driveway entrance will have a much shorter radius, such as 15 feet.