The M3 is the road south-west from London, serving the densely populated towns in the west of Surrey, the north of Hampshire and the south of Berkshire. It also carries traffic heading for the busy destinations of Southampton, Bournemouth and Poole via the A31, and the south west via the A303. All of these places have considerable demand for travel to and from London. As a result, the M3 is not especially long as major inter-urban motorways go, but it certainly isn't quiet.
For much of its existence, the M3 ran from the outer edges of London to Popham, west of Basingstoke, where it split to feed traffic into the A30 and A33. In the mid-1980s work started to extend it south, along the line of the A33, to reach Southampton, with the work progressing in sections until it ran both north and south of Winchester. The A33 Winchester Bypass had been built in the 1920s, making it one of the oldest sections of dual carriageway in the UK, and had intermediate junctions controlled by traffic lights. It was left to deal with all the M3's through traffic.
In the way of the M3's completion was St Catherine's Hill - a very ancient and very important hill fort, which the Winchester Bypass avoided by squeezing between the hillside and the river on a twisting path that was unsuitable for a motorway. There was no easy way around the obstacle.
The final section of the M3, which eventually opened in 1995, was built by carving an exceptionally deep cutting through high ground to the south east of St Catherine's Hill, leaving the ancient monument intact but destroying a picturesque and scientifically interesting bit of countryside. Its name stays in the popular conscience even today, such were the monumental protests that the construction of the road caused. Twyford Down is sometimes regarded as the beginning of the backlash against road building that saw construction of new roads in the UK fall out of favour with politicians of all colours for many years, and to this day the word 'motorway' tends to be avoided on new road schemes.
At its southern end, the M3 is fortunate enough to have its traffic effectively distributed by the M27, which kindly prevents all six lanes from piling into the northern suburbs of Southampton and is the sort of effective terminus that many other motorways probably ought to have.