The A9 is a major road running from the Falkirk council area in central Scotland to Scrabster Harbour, Thurso in the far north, via Stirling, Bridge of Allan, Perth and Inverness. At 273 miles (439 km), it is the longest road in Scotland and the fifth-longest A-road in the United Kingdom. Historically it was the main road between Edinburgh and John o' Groats, and has been called the spine of Scotland.
The road's origins lie in the military roads building programme of the 18th century, further supplemented by the building of several bridges in later years. The A9 route was formally designated in 1923, and originally ran from Edinburgh to Inverness. The route was soon extended north from Inverness up to John O'Groats. By the 1970s the route was hampered by severe traffic congestion, and an extensive upgrading programme was undertaken on the 138 mile section between Bridge of Allan and Inverness. This involved the bypassing of numerous towns and villages on the route, and the building of several new bridges, notably the Kessock Bridge which shortened the route north out of Inverness by 14 miles.
In the south the road's importance has been eclipsed by (1) the A90 across the Forth Road Bridge and the M90 motorway, which now link Edinburgh more directly with Perth, bypassing Stirling and Bridge of Allan as formerly important bridge points, and (2) the M9, which is now the main road between Edinburgh and Bridge of Allan. Between Edinburgh and Falkirk the old A9 route has been reclassified into the A803 and the B9080 amongst others; part of the route between Kirkliston and Maybury no longer exists as the area is now part of Edinburgh Airport. Between Falkirk and Bridge of Allan, the A9 survives as a more or less parallel road to the M9.
The link between the M9 and the A9, by Bridge of Allan, is the Keir Roundabout.
The A9's origins lie in the military roads building programme carried out by General Wade in the 18th century to allow deployment of forces in key locations within the Highlands. At this time there was already an existing road between Perth and Dunkeld, and between 1727 and 1730 a roadway was constructed between Dunkeld in Perthshire and Inverness.