The road between Edinburgh and Glasgow is one one the most important in Scotland, and has three very important phases in the 20th Century.
At the start of the twentieth century, there were two historic routes between Edinburgh and Glasgow, running roughly parallel for the 45 miles between the cities.
The southern route started from Edinburgh's old West Port (now the west end of the Grassmarket), and went through Mid Calder, Livingston, Whitburn, Harthill, Salsburgh, Newhouse, and Bellshill. With the road numbering in 1922, from Edinburgh-Mid Calder became A71, from Mid Calder to Bellshill was A705, and A725 west from there.
The northern route, starting at the West End of Edinburgh's New Town, passed through Newbridge, Broxburn, Bathgate, Airdrie and Coatbridge. With the 1922 numbering, this route became the A8.
These routes had competition from several different railways, and from the canals.
Due to later rebuilding, only about half of each route remains in anything like its original form.
The northern route, the old A8, can still be followed west from Bathgate as the A89. It's a single carriageway road, passing over a broad upland moor. Descending towards Glasgow, the road passes through Airdrie, Coatbridge
The Glasgow-Edinburgh road was almost completely rebuilt in 1923-28 (D.A. Donald, engineer) Forty miles of new road cost £2,400,000, with part funding from the cities of Glasgow and Edinburgh and the county of Lanark for the sections within their areas (other councils did not contribute) and a 75% grant from the Ministry of Transport. .
At the time there was not nearly enough motor vehicle traffic between Edinburgh and Glasgow to justify construction of a high-type motor road. Bonar Law, then Prime Minister, authorized the expense for unemployment relief and as a special favor to Scotland (although he was born in Canada, he was an Ulster Scot through and through).
The new road had a formation width of 100' to 120' (except through Harthill), and was designed throughout as a dual carriageway. However, most was only ever completed as a single carriageway, with the second carriageway being left as a grass strip. The only dual-carriageway built was at the Glasgow end, the surviving Old Monklands section of the A8, and about a mile south of Bathgate. The grade separated junctions on the Old Monklands (and possibly also the one at Bathgate) date from the early 1960s. The section from Newbridge to Gogar was upgraded to dual-carriageway in the early 1970s.
Apart from Harthill, all the major communities along the route were by-passed. Salsburgh, just west of Shotts, had the new road build close along the south side of the village, but new housing estates sprang up on the opposite side - there's still a 40 mph speed limit through Salsburgh, matched only at Ratho/Newbridge and Gogar on the outskirts of Edinburgh, and on the urban section in eastern Glasgow.
A section of the road in West Lothian also carries a 50 mph speed limit. The rest is NSL, i.e., 60 mph on single carriageway and 70 mph on dual carriageway. (The rule than NSL is 30 mph where there are streetlights doesn't really apply, since the parts of the road that remain open are all classified - apart from a half mile length west of Newhouse).
Starting in 1964, the Edinburgh-Glasgow road was progressively upgraded to Motorway.
The first section to be completed was the Harthill by-pass - removing the one significant bottleneck on the old road. The new section began just east of the present J5 (879641) and continued to the roundabout at Whitburn (946655). The diversion of the old road was also completed at this time, freeing the future alignment of the motorway.
The Harthill service area was designed into the motorway from the outset, but isn't shown on the earliest maps.
The next section of motorway was a westward extension, initially following the line of the old road around the hill at Kirk o' Shotts, and then on a new alignment to the present end of motorway at Newhouse. J5 and J6 were completed at this time, and the motorway continued to its present terminus on the A8 dual carriageway.
The old hill road over Kirk o' Shotts was upgraded, and was now used to link the two sections of the 1924 Glasgow-Edinburgh Road. The change from the wide dual-carriageway formation to the hill road is very marked.
Some Bartholomew maps show the motorway slewing southwards to a temporary terminus at the old Newhouse roundabout on the Glasgow and Edinburgh Road - but evidence on the ground suggests that the current motorway was built through to the A8 at Old Monklands. It's possible that it was initially opened as far as J6, but I think it most unlikely.
The third major section was an eastward extension from Whitburn to the Newbridge roundabout. The roundabout was built on ground where the Ratho-Queensferry railway crossed the old road, and was the temporary terminus of both the M8 and M9 for many years. (Some maps show that intermediate sections between Whitburn and Newhouse may have been opened early.)
The fourth section was west of the present J26, bypassing Renfrew and Paisley.
The trigger for motorway here was the closure of the old Glasgow Renfrew Airport, with civil operations moving to the present Glasgow Airport at Abbotsinch. The long straight of the motorway west of J26 roughly follows the line of the runway of the old airport. There is very little surviving on the ground
This road was numbered as A8(M) for a short time. When it became M8, junction numbers were allocated in sequence with the eastern section of M8, with no break in numbering. J6 at Newhouse was the westernmost extent of the motorway from Edinburgh, so the Braehead junction (now J26) was numbered as J7.
The original western terminus of this section was a junction on the level with the A8, west of Abbotsinch. The motorway fed straight onto the A8 towards Greenock, with the A8 from Glasgow ending at a T-junction right at the end of the dual carriageway.
The extensions west came thick and fast: The Erskine Bridge road, M898 and A898 came first, and the junction with the A8 was modified so that the M898 traffic went straight on, with a westbound exit leading to the westbound A8, and an eastbound exit from the A8 going round a loop to join the M8 eastbound. No other turning movements were possible.
After the M8 opened through to J31, the temporary connections at the A8 crossing were removed. I believe there is now no trace on the ground.
Starting around 1965, many of the old industrial areas fringing Glasgow's centre were earmarked as Comprehensive Development Areas (CDAs). Apart from small breaks, the brownfield sites formed an almost continuous ring around the central area, and local politicians and press were quick to realise the potential for a motorway ring road.
The original plans called for a complete inner ring road, with 7 radial spurs.
In the event, only the northern and western limbs were completed, and only three of the planned radial routes.
More details and photographs of the Inner Ring Road can be found on M8 Glasgow.
The final Glasgow section to be completed was from the city centre out to Ballieston. This followed the old route of the Monklands Canal, and is therefore nearly level apart from a sharp rise
Both the M8 and M9 were designed to be extended east from Newbridge, meeting at a junction SE of Newbridge and continuing to Edinburgh. When the route was finally chosen, only the M8 was extended as planned, and the M9 was extended southwards from Newbridge to a new junction on the M8.
Still to be upgraded to motorway is the Old Monklands section of the A8 Edinburgh-Glasgow road.