For most of its life, the M90 and its friend the A823(M) were all alone. The route connects Edinburgh, via the Forth Road Bridge, to Perth and the north and north-east of Scotland. Since it first opened in 1964, it has been separated from the rest of the motorway network by the Forth Road Bridge, which was part of the A90 and not a motorway.
A second crossing of the Forth is now under way, which will be classified M90, and in preparation for its opening a formerly anonymous spur of the M9 has already been numbered as part of the M90. That means that the M90 now does technically connect to the rest of the motorway network - though it isn't actually continuous yet!
At the northern end of its route the motorway splits as it approaches Perth, causing an interesting feature. Heading north, the spur route to the A9 is longer than the remaining motorway straight ahead to the north east.
The M90 is only just a motorway. It has some ridiculously sharp corners (including what is reputedly the tightest curve on the motorway network), and large parts of it lack hard shoulders, instead just having frequent lay-bys. This can, of course, be excused given the challenging geography.
Ronnie Land, who worked on the scheme in the 1960s, notes that the three-way motorway interchange south of Perth required the removal of some 900,000 cubic metres (31,783,000 cubic feet) of material. The minimum curve radius used on the route is 694m (2,278ft); a minimum of 914m (3,000ft) was standard practice at the time. Likewise the maximum gradient then permitted by design standards was 1 in 30 (1 in 20 for hilly areas); some of the M90's slip roads are a little steeper than 1 in 17.
In the early 1990s, mass renumbering of roads up the east side of Scotland was carried out to give the route from Edinburgh to Aberdeen one continuous number, A90. This did some strange things to the area south of Perth where the M90 splits.
Before the change, the M90 went up the west side of the fork to meet the A9. As the A90's traditional northern terminus was Perth, this was the end of the line. The A85 passed through Perth on the east-west axis, and went on to Dundee, and so the motorway to the east of Perth, over the Friarton Bridge, was M85. The change caused the M90 designation to switch from the west to the east of Perth, eating up the entirety of the very short M85, in order to connect the motorway to the extended A-Road. The bit it left behind is now signposted '(A9)' northbound and '(M90)' southbound. This means that the northernmost motorway in the UK has no number.
The numbering as it stands now makes more sense than before: the M85 was always the more popular route, and the junction where the three sections of motorway meet south of Perth gives priority to traffic between the south and Dundee.