Best type of hedge for noise?

Unfortunately, no hedge will significantly reduce traffic noise. What you need is density (or a vacuum!). A good example would be a earthen berm. Like an inverted ditch to absorb the (mainly low frequency) sound from the traffic. Objects that are less dense, eg plants/trees will not absorb low frequency sound at all. A good thick, dense Leylandii hedge will absorb some mid to high frequencies, but not massively. Added to that some of the emerald green varieties can grow up to 3ft a year also causing them to need regular pruning to keep them in shape. If you prune little and often when they are at the desired height you will prevent the risk of over pruning and getting back to the brown dead foliage which will never go green again. Perhaps another solution would be a tall dense bamboo hedge, it would PRODUCE noise as well as being relatively dense. The noise it produces would be a distraction from the road noise atleast. The only real problem with bamboo is that it spreads like wild fire, it would need to be contained underground, usually in plastic pots, but for a hedge something more robust to stop it spreading. Sorry there are no easy answers here - a 6' high brick wall would definitely work better, but may not be the 'look' you are aiming for.

A dense evergreen hedge is probably your best option, such as a leylandii or laurel. If they are maintained regularly (once or twice a year) into the required height and thickness they will eventually have a good density. Hedges always look nicer and are a lot better for the environment than walls or fences. Another technique used to block out sound from a noisy road is to put a water feature in the garden, the sound this gives off draws the attention away from the road.