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 is removed if no longer needed. Clean, legible, properly mounted devices in good working condition command respect. They have businesslike appearance that implies that they are official and enforced—thus earning the respect of motorists. In addition to physical maintenance, functional maintenance is required to keep traffic control devices current. The fact that a device is in good physical condition should not be a basis for deferring needed replacement or change. A device must be functionally sound or it has outlived its usefulness. Furthermore, carelessly executed maintenance can destroy the value of a group of devices by throwing them out of balance. For example, replacement of a sign in a group or series by one that is disproportionately large may tend to depreciate others in the vicinity; maintenance must be functional a s well as physical to guard against such occurrences.

Uniformity of traffic control devices simplifies the task of the road user because it aids in instant recognition and understanding. It aids police courts and road users by giving everyone the same interpretation. I t aids public highway and traffic officials through economy in manufacture, installation, maintenance, and administration.

Simply stated, uniformity means treating similar situations in the same way. The use of uniform traffic control devices does not, in itself, constitute uniformity. I n fact, using a standard device where it is not appropriate is as objectionable as using a nonstandard device.

Engineering Study Required

The decision to use a particular device at a particular location must be made on the basis of an engineering study a t the location. Thus, while this Manual describes the application of the various devices, it is not intended as a substitute for engineering judgment. Nor is it intended to discourage the imaginative application of the standards and principles which the Manual prescribes. Both engineering judgment and imaginative application are essential to true uniformity.

Furthermore, the National Joint Committee on Uniform Traffic Control Devices has provided for limited and rigidly restricted recognition of proposals for experiment or research which it believes may hold promise for advances in this ever-developing field. The Joint Committee requests that it be informed of all new significant research and development which may affect future designs, standards, and uses. The Committee will periodically review the standards with the objective of revising those which 3