The M9 is a fairly nondescript motorway, despite its important number: it forms part of the trunk route from Scotland's capital city to the far north of the Highlands, but the A9 takes all the really interesting parts of the journey and the motorway is left to cover the first few miles through the central belt.
With two lanes each way for most of its length, it doesn't travel very far and on its own it doesn't lead anywhere spectacularly large or important outside Edinburgh. Its main feature is the contrasting scenery between the peaceful grounds of Linlithgow Palace and the industrious grey of Grangemouth and Bo'ness.
Once upon a time the M8 and M9 both terminated at Newbridge Roundabout on the A8 just west of Edinburgh - not good. In 1992, the M8 was extended west into Edinburgh from a point about two miles south of Newbridge, and as part of the scheme, the M9 was improved and now continues under the roundabout and along the road the M8 left behind. It new terminates on its sibling at a large free-flowing interchange, which makes the whole situation west of Edinburgh considerably tidier.
The M9's only spur, a short dual carriageway headed for the Forth Bridge, has grown considerably in importance and stature. Until 2007, it gave up a couple of miles short of the bridge, and unburdened its considerable traffic load onto a narrow single carriageway road, the A8000. There was great relief when it was extended to reach the A90, and the two-mile extension to this anonymous spur road was one of Scotland's most eagerly awaited transport projects in recent years. It has now been reclassified as part of the M90 in readiness for the opening of the new Queensferry Crossing over the Forth, and will become the new southern end of a major motorway once the new bridge opens and connects it up to the rest of the M90.