A large number of supplemental advance warning devices have been used by roadway agencies to inform motorists of unusual geometric, operational, or traffic control features. The use of a device by an agency does not imply that it is a viable or desirable device to use for identified deficiencies. The following concerns should be considered prior to the installation of any device not specified in MUTCD:
• Many warning devices are attempts at political, inexpensive, and/or quick solutions to totally inappropriate roadway conditions. The proper countermeasure for many of these conditions is to correct the fault rather than installing an additional motorist warning. Installing a supplemental warning device should be considered a temporary countermeasure until the inadequate roadway conditions can be corrected.
• MUTCD provides guidance on the proper placement of traffic control devices to provide adequate time for motorists to perceive, identify, decide upon, and perform any necessary maneuver. Section 2C-3 provides guidelines for the minimum placement distances of warning signs, while Sec. 4D.15 specifies the minimum continuous visibility distances that should be present for motorists approaching a traffic signal. The inability to provide the minimum visibility distance is one indication of the need to install an advance warning sign. Guidelines on the height and lateral location of signs are summarized in Fig. 2A-1 in MUTCD [2]. The guidelines of Part 2— Signs of MUTCD should be followed for the installation of all traffic signs.
• Section 2C.03 of MUTCD states that warning signs shall consist of a black legend and border on a yellow background [2].
• Section 2C.02 of MUTCD permits the design of warning signs for special conditions [2]. These signs should, however, be constructed with clear and concise verbal messages. Letter legibility and size, combined with placement, must provide a clear meaning and provide ample time for response. Section 1A.10 of MUTCD provides an approval process for new symbols and does not permit the use of symbols that are new or unique and, thereby, not readily understandable by the motorist [2]. The only exception to the provision of nonstandard symbols is where minor modifications to MUTCD symbols are necessary to adequately describe specific design elements of the roadway. An example of a permitted symbol modification is displaying a curve on “Intersection Warning Signs” (W2-2) if the side road occurs in the vicinity of a horizontal curve. Devices that use symbols not contained in MUTCD, or in Standard Highway Signs, are nonstandard devices [2, 9].
• Warning devices should have the same silhouette shape as the device shape. For example a 36-in X 36-in (915-mm X 915-mm) diamond warning sign mounted on a 48-in X 48-in (1220-mm X 1220-mm) square piece of plywood would not satisfy the shape requirement. Dawn and dusk light conditions, fog, and other poor-visibility situations can result in interpreting the warning sign as a guide sign.
• Section 4K.03 of MUTCD permits the use of hazard identification beacons to supplement an appropriate warning sign or marker [2]. The hazard identification beacon consists of one or more sections of the circular yellow traffic signal head indication with a visible diameter of not less than 8 in (200 mm). MUTCD prohibits the placement of the beacons within the border of the sign except when used with a School Speed Limit sign. If two beacons are used, they should be alternately flashed at a rate of not less than 50 nor more than 60 times per minute.
• Unique situations in the roadway environment can result in the need for changes or additions to MUTCD. Section 1A.10 provides the procedure to be followed for consideration of a new device to replace a present standard device, for additional devices to be added to the list of standard devices, or for revisions to recommended application. Agencies that encounter the frequent need of a unique application are encouraged to request permission to experiment from the Federal Highway Administration, Office of Transportation Operations (HOTO), 400 Seventh Street S. W., Washington, DC 20590.
