This synthesis will be of interest to traffic engineers, highway design engineers, and others concerned with the operation, safety, and design of the roadway environment. Information is presented on a variety of advance warning devices that are used by state and local agencies to advise motorists of unusual or dangerous road conditions. This synthesis describes the state of the art with respect to these devices. This report of the Transportation Research Board describes more than 340 such devices that have been applied and are in keeping with accepted traffic engineering principles, but are not presently included in the Manual on Uniform Traffic Control Devices (MUTCD). Both active and passive warning devices are presented in the following general categories: environmental conditions (e.g., snow and ice), heavy vehicles, school zones, pedestrian crossings, rail-highway grade crossings, roadway cross sections, congestion and queues, and others. The purpose and characteristics of each device are described, including any information available on their effectiveness, cost, or other features. A drawing of each device is shown to aid the reader in visualizing the device.  The report for this topic can be purchased at https://www.trb.org/news/blurb_detail.asp?id=3371