M1 in ETS 1 Edit
The M1 was first introduced in the Gold Edition of Euro Truck Simulator. It's the only in-game road running north from London, the capital. Unlike the real-life motorway, the M1 in this game spanned all the way to Newcastle, the northernmost featured city. In addition, there's a TOTSO for M1 traffic at the junction with M62 from Manchester.
M1 in UKTS and ETS 2 Edit
SCS Software released an expanded map of United Kingdom in their next trucking game, UK Truck Simulator. The M1 in that game was curtailed to run between the M25 and the M62, making the road slightly shorter than its real-life counterpart.
As the new map featured more cities and roads, the M1 became understandably longer, and features 4 major road junctions, 2 minor road junctions and 3 gas stations. However, the number of cities directly linking the M1 reduced from 2 to 1, with the newly featured Sheffield being the only city having roads directly from the motorway.
This setting is unchanged when SCS released Euro Truck Simulator 2.