Coal generated electricity has been replaced by gas and wind.
I am unclear what you are proposing to replace the M1 with?
Road vehicle usage has barely grown in 20 years. Rather like the rail network, usage grows when capacity is increased and little expansion has taken place since the 1990s.
Road growth forecasts are fanciful - based on extrapolating three consecutive years of slight growth over decades. The actual figure is still barely changed on 2000. But more importantly HS2 is the biggest strategic transport project since the motorway programme and makes no meaningful impact on road transport.
Whilst there's been some replacement of coal powered power stations with other methods of generation some have gone as we now use less energy than we did in the past due to being more energy efficient.
The same could be true of travel. Some would be replaced by rail, however others just wouldn't need to be replaced if the roads were more efficient. In a 12m length of road you can fit 2-14 people of they use cars, however use a bus and you can get 70 people in the same space.
That means instead of needing 5 lanes of traffic you can replace it with a single lane. However that's fairly simplistic in that it doesn't take into account stopping distances. However given that buses need slightly longer to stop, whilst 2 cars need less distance but that distance to be double, the overall difference isn't going to be that much different.
A lot of trips are fairly short and so the impact of not driving would be fairly small.
For instance 22% of all trips are less than 1 mile, however ~19% are made by a mode other than walking (the vast majority of which are driving). Likewise 56% of all car trips (by number) are less than 5 miles. Those distances lead themselves to other modes fairly well (less so walking once you get above about 2 miles, but even then that's not totally outrageous to do).
Those short distance tend to happen in cars as people "need" a car for other purposes. By improving rail (including HS2) you reduce that "need" and so reduce the likelihood of someone owning a car in the first place.
If you reduce the likelihood of them owning a car then they stop driving those short distances as the cost of owning a car increases to the point is not viable.
Few people who own a car will be paying less than £2,000/year in purchase/deprecation, maintenance, insurance, finance, replacing tyres and other costs associated with ownership which doesn't include fuel costs.
£2,000 gets you a fair distance on bus/train season tickets. Even if you throw in some car hire and taxis. If you are doing much less than the average (circa 7,000 miles a year) milage in a car then chances are you are paying quite a high figure on a per mile basis.
Where have I said that we should replace the M1?
I've given the distance of road which could be closed to result in a net zero carbon emission of we built HS2. However chances are it wouldn't be just close 350 miles of the total strategic road network, is more likely to be not need to build a given road improvement or reduce the size of a road (say from 3 lanes to 2) which could be viable if alternative travel options existed. Whether that's better local facilities like improve local trains, buses, cycle lanes and the like or whether that's building a new intercity line, in any case being able to compress the amount of roadspace required for any given number of people to travel.