The Design Method is based on the use of curve geometry data obtained from files or as-built plans. This method is suitable for evaluating newly constructed or reconstructed curves because the data are available from construction plans.

The procedure for implementing the Design Method consists of three steps. During the first step, curve geometry data are obtained from files or plans. During the second step, the measurements are used to compute the advisory speed. During the last step, the recommended advisory speed is confirmed through a field trial run, if or when the curve exists. Each of these steps is described in the remainder of this subsection.

Step 1: Obtain Curve Geometry.

Consult the appropriate files to obtain the radius, deflection angle, and superelevation rate for the curve. If the curve is circular, the “total curve deflection angle” is equivalent to the “curve deflection angle,” as used in TCAS. The total curve deflection angle equals the deflection angle in the two intersecting tangents.

If spiral transition curves are included in the design, obtain the radius and superelevation rate data for the central circular curve. The total curve deflection angle is the same as defined in the previous paragraph. The curve deflection angle represents the deflection angle of the central circular curve.

If compound curvature is used in the design, obtain the radius and superelevation rate data for the sharpest component curve. The total curve deflection angle is the same as defined in the first paragraph. The curve deflection angle represents the deflection angle of the sharpest component curve.

Obtain the aforementioned data for both directions of travel if the road is divided or if conditions suggest the need for separate consideration of each curve travel direction. When two or more curves are separated by a tangent of 600 ft. or less, one sign should apply for all curves. However, data for each curve should be obtained in this step.

Step 2: Determine Advisory Speed.

The data obtained in Step 1 are entered in TCAS in the section titled Alternate Input Data. If a reasonable estimate of the 85th percentile tangent speed is not available, the speed limit can be used in TCAS to estimate the 85th percentile tangent speed.

When two or more curves are separated by a tangent of 600 ft. or less, one sign should apply for all curves. However, each curve should be evaluated separately in this step. The advisory speed plaque should show the value for the curve having the lowest advisory speed in the series.

Step 3: Confirm Speed for Conditions.

During this step, the appropriateness for the advisory speed determined in Step 2 is evaluated. The activities conducted during this step are the same as those discussed in Step 3 of the Direct Method except that the need for an advisory speed plaque is checked using the information in the Traffic Control Device Guidance section of TCAS.