A curve warning sign with a 40 mph advisory plaque indicates that:

Correct answer: C – 40 mph is the recommended operating speed for the curve under favorable conditions
An advisory speed plaque displays the recommended operating speed for the condition shown on the warning sign. Here, 40 mph is the advised speed for the curve under free-flowing conditions. Weather, road surface, traffic, vehicle characteristics, or cargo may require a commercial driver to travel more slowly.
Source: Manual on Uniform Traffic Control Devices for Streets and Highways, 11th Edition with Revision 1 — § 2C.59, “Advisory Speed Plaque (W13-1P) and Confirmation Advisory Speed Plaque (W13-1aP)”


