Starbuckone, highway visibility is exactly why fences are restricted to 1m by the highway under permitted development. The planner will look at whether the fence would obstruct highway safety and whether it would affect the appearance of the area. However as hedges are not built form, they do not fall under the Planning Acts and can be any height. But remember there is now high hedge legislation as well, so if your hedge causes a nuisance you could theoretically be asked to reduce its height.