Three methods for establishing curve warning advisory speeds are described in this section. Any one of the following three methods can be used to determine the curve advisory speed.

Issues with Ball-Bank Indicator.

Historically, the ball-bank indicator has been used to establish the curve advisory speed. However, this device is susceptible to forces that are not a result of road curvature (e.g., bounce due to rough pavement, jerk due to steering corrections, slip due to variation in pavement friction supply, etc.). These forces come and go while traveling along the curve and cause the ball-bank indicator to produce readings that can vary randomly by several degrees. All too often, one of these random peak readings is mistakenly used as the basis for determining the advisory speed. This characteristic of the ball-bank indicator is one reason that advisory speed is not uniform among curves of similar geometry. For this reason, the three methods described in this section are not based on a specific threshhold ball-bank reading (e.g., 10 degrees). The methods are based on the same criteria and will provide a uniform and consistent display of advisory speed among curves, regardless of which method is used. For all three methods, the advisory speed is defined as the average speed of free-flowing trucks.

Steps for implementing each method are described later in this section. The

Direct Method

is based on the field measurement of curve speed. The

GPS Method

is based on a single-pass survey using a GPS receiver and software to compute the curve radius and deflection angle. The

Design Method

is useful when the radius and deflection angle are available from as-built plans.

Compliance Period.

In order to allow the districts ample time to acquire the necessary equipment to perform one of the three methods described in this section, the ball-bank method may be used to determine the advisory speed of curves until January 1, 2013. The steps for determining the advisory speed using a ball-bank indicator will remain in this section until the compliance date of January 1, 2013 has passed. After that date, curve advisory speed signs installed on the state highway system shall be established by one of the methods described in this section. All existing curve advisory speed signs shall be verified to be in compliance with any one of the three methods described in this section (and changed if necessary) by a target compliance date of January 1, 2015.