Horizontal Curve Sight Distance

Horizontal Curve Sight Distance

Once you have a radius that seems to connect the two previously disjointed sections of roadway safely and comfortably, you need to make sure that you have provided an adequate stopping sight distance throughout your horizontal curve.

Sight distance can be the controlling aspect of horizontal curve design where obstructions are present near the inside of the curve. To determine the actual sight distance that you have provided, you need to consider that the driver can only see the portion of the roadway ahead that is not hidden by the obstruction. In addition, at the instant the driver is in a position to see a hazard in the roadway ahead, there should be a length of roadway between the vehicle and the hazard that is greater than or equal to the stopping sight distance. See figure 1.0 below.

picture of sight distance around an obstruction

Figure 1.0: Sight Distance

Because the sight obstructions for each curve will be different, no general method for calculating the sight distance has been developed. If you do have a specific obstruction in mind, however, there is an equation that might be helpful. This equation involves the stopping sight distance, the degree of the curve, and the location of the obstruction.

M = (5730/D)*(1 - cos(SD/200))

Where:
M = Distance from the center of the inside lane to the obstruction (ft.)
D = Degree of the curve. Where R = 5730/D
S = Stopping sight distance (ft)
R = Radius of the curve (ft)

Once your rough design has been adjusted to accommodate the sight distance restrictions, and you are satisfied with the aesthetic and financial consequences of your superelevation scheme, you can begin to polish your design into its final form.