While the advantages of uniformity far outweigh the disadvantages, there are some undesirable effects when complete uniformity is maintained. One of the principal disadvantages is that strict uniformity may result in the failure to adopt an improved device or procedure simply because it is not in common use. In addition, total uniformity would require the specification of a separate traffic control device for every conceivable roadway geometric and traffic operational condition. This would be a monumental task that undoubtedly would still not cover every situation, while simultaneously increasing the size of MUTCD with devices of limited application.
This difficulty is recognized in MUTCD, which indicates that warning signs other than those specified in the manual may be required under special conditions [2, Sec. 2C.02]. MUTCD requires exercising good engineering judgment in determining the need for other warning devices. It also mandates that the innovative devices be understood easily by the motorist. Ensuring that warning signs are easily understood necessitates that they be of standard shape and color and that the legends be unambiguous and brief. Establishing the need for distinct warning devices can be accomplished by identifying when standard devices do not properly address unusual conditions. While these conditions are unusual, they can typically be classified into the same use categories that are appropriate for standard warning signs. The Traffic Control Devices Handbook [3] identifies the following uses of warning devices:
• To indicate the presence of geometric features with potential hazards
• To define major changes in roadway character
• To mark obstructions or other physical hazards in or near the roadway
• To locate areas where hazards may exist under certain conditions
• To inform motorists of regulatory controls ahead
• To advise motorists of appropriate actions
The need to provide advance warning for unusual roadway, roadside, operational, and environmental conditions has resulted in the development of a wide diversity of devices. The majority of these devices can be categorized as warning signs containing different symbols and legends. Other warning devices include flashing beacons, rumble strips, pavement surface treatments, and pavement markings. Device complexity ranges from simple passive warning signs to devices that are activated by vehicle speed, headway, or presence on one or more approaches to a potentially hazardous roadway element. Further information on supplemental warning and rumble strips can be obtained from the National Cooperative Highway Research Program (NCHRP) publications Synthesis of Highway Practice 186: Supplemental Advance Warning Devices and Synthesis of Highway Practice 191: Use of Rumble Strips to Enhance Safety [4, 5].
