The Direct Method is based on the field measurement of vehicle speeds on the subject curve. The procedure for implementing the Direct Method consists of three steps. During the first step, speed measurements are taken in the field. 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. Each of these steps is described in the remainder of this subsection.

Step 1: Field Measurements.

Measure the speed of 125 or more free-flowing passenger cars at about the middle of the curve in one direction of travel. Normal speed checks should be discontinued after two hours if radar is used or after four hours if a traffic counter that classifies vehicles by type is used even if 125 cars have not been timed. A free-flowing vehicle will be at least 3 seconds behind the previous vehicle. A radar speed meter can be used for this purpose.

Repeat the measurements for the opposing direction 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 feet or less, one sign should apply for all curves. However, each curve should be surveyed separately in this step.

Compute the arithmetic average of the measured speeds for each direction of travel at each curve studied. Also, compute the 85th percentile speed for each direction and curve.

Step 2: Determine Advisory Speed.

Multiply each of the average speeds from Step 1 by 0.97 to obtain an estimate of the average truck speed for each direction of travel. The advisory speed for each direction of travel is then computed by first adding 1.0 mph to the corresponding average and then rounding the sum down to the nearest 5 mph increment. This technique yields a conservative estimate of the advisory speed by effectively rounding curve speeds that end in 4 or 9 up to the next higher 5 mph increment, while rounding all other speeds down. For example, applying this rounding technique to a curve speed of 54, 55, 56, 57, or 58 mph yields an advisory speed of 55 mph.

When two or more curves are separated by a tangent oof 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 of the advisory speed determined in Step 2 is evaluated. As an initial task, the need for an advisory speed plaque is checked. A representative 85th percentile on tangent sections of the roadway is needed for this check. It can be measured using the procedure described in Step 1 or estimated from the regulatory speed limit. If it is measured, the point of measurement should be at least 8 seconds travel time from any curve in either direction. The 85th percentile tangent speed and the 85th percentile curve speed (from Step 1) are used with Figure 5-2 to determine the need for an advisory speed plaque.

A second task involves a field evaluation of curve conditions. The evaluation includes consideration of the following factors.

The unexpected geometric features noted in the third bullet may include:

A final task involves a test run through the curve while traveling at the advisory speed determined in Step 2. The engineer may choose to adjust the advisory speed or modify the warning sign layout based on consideration of the aforementioned factors. The advisory speed for one direction of travel through the curve may differ from that for the other direction.