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V.4.17.13 (Cont'd)Amendments No. V-2/85 (July)the solution is to level off before the exit or use a strictlyperpendicular head-wall.The designer should be aware of illusion, either to exploitor counteract its effects in the interests of good design.4.17.14 Long term appearanceEvery effort should be made to ensure that attractivestructures remain so for the whole of their useful lives. This can beachieved by using durable materials which will weather well and notdeteriorate significantly with time. Less durable materials, if usedat all, should be confined to components which can be readilyreplaced.Sensible detailing to reduce the chance of subsequentspoiling of surfaces by natural staining, accidental damage ordeliberate vandalism is essential to good long-term appearance.Near-horizontal surfaces likely to gather dust in dry weather shouldbe sloped to direct wash-off away from vulnerable faces. Jointsshould be carefully designed and executed to prevent leakage, andprecautions should be taken to limit any staining which might ensueif, despite these precautions, leakage should occur. Figures 4.17.21and 4,17.22 show some examples of the effect of good and bad detailingon long-term appearance.Finally, of course, the initial and long term appearance ofstructures will be greatly enhanced by, and is indeed largelydependent upon, sound, carefully supervised construction practice anda systematic programme of regular maintenance thereafter.4.17.15 The Advisory Committee on the Appearance ofBridges andAssociated Structures (ACABAS)The ACABAS has been set up to enquire into the visualmerits, general amenity value and related environmental factors of allproposals to construct bridges and associated structures over, under,on or adjacent to public roads in the Territory.The designs of all such structures should be approved byACABAS before construction. Submissions to ACABAS should as far aspossible be made at the preliminary design stage, to avoid effortbeing wasted on the detailed design of an unsatisfactory concept.Full details of ACABAS, including its terms of reference andits membership are given in Lands and Works Branch Technical CircularNo. 7/84, which is reproduced in Appendix 4.17.1Experience since promulgation of the circular has shown thatphoto-montages are very effective for illustrating the finalappearance of a bridge in relation to its surroundings. ThereforeACABAS now accepts appropriate photo-montages instead of perspectiveviews or models as required by the circular.When repairs or alterations are proposed that willsignificantly affect the appearance of an existing bridge orassociated structure, details of the proposal should be submitted toACABAS for approval prior to implementation.