The increase in traffic levels over the last two decades has generated the need for measures to counter the resulting noise. In planning future roads it will be possible to choose alignments where noise screening measures will only be required as an exception, and then planted mounds carefully integrated into the landscape can be used. However in planning the majority of older roads noise protection was not considered and with the retrospective introduction of accoustic screening problems arise due to shortage of space, in particular in mountain areas. Numerous types of accoustic screen wall have been built, using concrete, metal, timber, plastics and even glass. The visual impact of such structures soon led to the desire to combine them with planting. First attempts in Austria involved crib wall structures using precast concrete units. Combining these with woody species not only led to an improvement in appearance, but also to wider technical benefits due to the stabilizing effects of the planting. Disadvantages resulted from the large surface area of concrete which tended to reflect rather than absorb sound. Today there are around 80 types of concrete units available which can be built up and planted. The next step was an attempt to create space saving earth structures which did without concrete units altogether. For these composted refuse was the substrate predominantly used as its coarse, loose structure absorbed sound better than other materials. Mesh or mats were used to stabilize these constructions along with central metal or concrete beams which were not visible from the outside. However the composted refuse proved to be high in phytotoxic substances and in addition there was a high loss of material within a relatively short time. Further developments first required new, more suitable natural substrates to be found. Certain mixtres of bark compost and living topsoil with the addition of substances furthering the development of soil micro-organisms have not only been found to fulfil all biological requirements but also to have improved sound absorption properties. The newest types of accoustic screen walls allow simple on-site construction as well as prefabrication which means that ready planted walls can be transported to site. Attempts to construct accoustic screen walls using only pallisades of living woody material are likely to have only limited success and application in that the accoustic screening of plant material alone is slight. All the above types of accoustic screen wall represent extreme habitats for plants: the volume of rooting medium is always small and with the use of concrete units there are additional problems of possible screening of precipitation and the overheating of the planting substrate. It will therefore usually be necessary to provide an irrigation system, although trials are underway using droughtres....
