That was one of the reasons put forward for HS2 going cross-country compared to HS1 which followed existing motorways and railways wherever possible. But I think it's a pretty minor issue to be honest. The land area caught in between the railway and motorway is not that big, and besides HS2 then decided to enclose the railway within a very wide strip of landscaping, thus making the land just as unusable anyway. In the end there may have been other reasons why the route was chosen - building next motorways can be problematic in all sorts of ways.
But there are two general points to be drawn from this. Firstly, properly think through decision making - particularly consider whether a problem is actually that critical, and think about the consequences of the decision (in the case of HS2, the high design speed, crazy no-maintenance specification and landscaping massively increased the size and cost of the project for marginal gain). And the second is if it ain't broke don't fix it. HS1 was built reasonably efficiently but HS2 seemed to disregard the experience gained and developed a highly bespoke project.
I would argue a major difference is the size of population centres that the two motorways go through. The M20/M2/A2 mostly skirts round anything large apart from Ashford and Ebbsfleet, before diving into a tunnel to get into London.
Whereas the M1 is going through Edgeware, Luton, Milton Keynes. The M1 doesn't go to Birmingham, so presumably you then follow the M6 corridor, so you're going through Coventry.
As the document
@FMerrymon links to, the mitigations required for such a route would end up costing more than the relatively direct route that was chosen.
Another thing to consider is that the current route of HS1 is not the originally proposed one. It was supposed to go via Tonbridge and enter London from the South East, so not following the motorway corridors.
That is quite interesting but the maps are very unclear and it’s still not really clear they properly considered the M40 or M1 routes. There’s no detailed design in terms of saying “we’d need a tunnel here”. Or saying why they couldn’t follow the M40 as opposed to the chiltern mainline south of Bicester.
It's a summary of work done previously. It explicitly says they considered the M1, M40, and Chiltern routes. They're not going to go into detailed design decisions. If you want more detail, try doing a Freedom of Information Act request for the 2009 work they mention.
But one of the ways of saving money, is to sift out the options early. Once you have enough data to show that a route is unsuitable, you don't waste any more resources on those.
And only appear to be comparing them in relation to Birmingham, whereas the M1 corridor option is particularly advantageous to East Midlands, Sheffield and Leeds
When the directed aim of the project is London to the West Midlands, it's unsurprising that options are going to measured against how they get to Birmingham.
