signs may be sited where they will not receive adequate illumination from headlamps, and it might be prudent to provide direct lighting regardless of the regulatory requirements. Examples include signs mounted unusually high above the level of the carriageway, on the off side of the road or at the entrance to a side road. Retroreflection is also less effective where the sign is presented at a large angle to the direction of oncoming traffic. Modern microprismatic materials can achieve high luminances for many drivers in defined situations, but not for all drivers in all circumstances. However, some are designed to produce luminances little better than that of traditional beaded materials. Where regulatory signs on lit roads are exempted from the requirement to be directly lit, high-performance microprismatic sheetings (achieving a minimum performance index of 3.0 when rated in accordance with BS 8408:2005) are recommended.
2.7 Some regulatory signs, including time plates indicating parking controls and those intended for pedestrians, need not be directly illuminated or reflectorised. These are listed in Schedule 17 Item 9.
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