The Low Clearance sign indicates the maximum amount of overhead clearance at low bridges, underpasses, and under other structures. Such signs must be used at all points where the clearance is less than 15cm greater than the maximum height of a vehicle plus it's load permitted under Provincial Law. In any case it must be used where the clearance is less than 4.3m. All vertical distances displayed on Low Clearance signs should be rounded to the nearest 0.1m less than actual.

The Low Clearance sign is installed to indicate a low clearance ahead. The WA-27 sign is installed on the overhead structure immediately above the centre line of the pavement of each lane.