Apart from the obvious danger areas, for example, school zones, narrow side streets and the like. I did the first IAM observed drive today - two hours long, and for those two hours I held up the traffic constantly, as my observer "forced" me to observe the 30mph limit along a selection of A roads. 40mph wasn't too bad, but at 30mph you are very clearly the obstruction, much to the annoyance of others. Even the buses and lorries were unhappy with me. Try it sometime - be disciplined enough to stick to the 30mph limit for a reasonable period and I'm sure you'll see what I mean.