Appendix CApplication of warrants and guidelines#
Traffic operation is facilitated by efficient control devices, but these devices must be selected and used following scientific investigation, not subjective assessment.
A proper evaluation embraces the measurement of certain factors, such as traffic volume, and a close engineering study of the environment to ascertain firstly, if there is a need for control in a particular situation, and secondly, the type of control device which should be used.
Long experience has established the respective conditions under which the many traffic control devices are justified. These conditions, called warrants or guidelines, may comprise quantitative figures or other general requirements at the site concerned. Established warrants and guidelines for the installation of control devices are prescribed in the Manual.
The use of warrants and guidelines is to ensure that:
- a)control devices are installed where the need has been proven, and only in such situations
- b)the most efficient treatment is provided for any given set of conditions, and
- c)standard treatment is employed at similar situations.
However, even if a formal warrant or guideline is satisfied in a particular instance, it does not necessarily follow that the relevant traffic control device should be installed on that basis alone. The justification for the installation of a particular device often involves a number of elements; some incapable of being expressed in quantitative terms. To attempt to include all such elements in formal warrants or guidelines would be impractical, and therefore the final decision must involve proper engineering judgement.
There will occur, in practice, many cases where a particular form of control would be best suited to the local conditions, although the requirements of the formal warrant or guideline may indicate otherwise. In some cases, it may be advisable to refrain from imposing a particular type of control, although the prescribed warrant or guideline may be met. In other instances, some form of control may be justified where there is no applicable warrant or guideline. Indeed, objective investigations will, in practice, frequently indicate that the most effective treatment should be constructional works, such as improvement to visibility or widening of pavement.
In all cases, engineering judgement must be used in assessing the need for treatment at any location. Warrants and guidelines set out in the Manual should be regarded as the means of exercising this judgment and selecting the proper treatment, rather than as a substitute for it.
However, it is emphasised that warrants and guidelines have been established after long experience and careful study and should not be departed from unless the necessity to do so can be fully substantiated.
Warrants and guidelines for particular traffic control devices are included in the relevant Part of this Manual.