The issue is the journey time for passengers on the route as a whole, and costs of the putative ‘via Hetahrow’ Route, ie:
Euston - OOC - Birmingham / NW / NE / Scotland destinations as HS2 will provide, versus
Euston - LHR - Birmingham / NW / NE / Scotland destinations on a putative alternative route via LHR instead of OOC (which isn’t going to happen).
The questions are, therefore:
Would the OOC route generate more or less traffic than via LHR?
What is the additional journey time for going via LHR vs OOC?
How many passengers are affected by this journey time increase?
How many passengers would have been expected to use OOC will have extended journey times as a result of a station at LHR instead?
What are the journey time savings for passengers who wish to access LHR from HS2, and how many passengers make that saving?
How much extra does it cost to go via LHR than OOC?
I don’t have access to the detailed modelling, but as I understand it:
OOC generates far more traffic for HS2 than an LHR station would, because of the connections with the GWML, Crossrail, London Overground and traffic from the local area (with significant development about to happen).
The additional journey time is in the region of 2-3 minutes, which would be experienced by all Passengers from central London to the Midlands and North via HS2
A significant number of ‘OOC’ passengers would have signiifcantly extended journey times if LHR is used instead of OOC (in many cases, sufficiently extended to be a disincentive to travel)
The journey time saving for LHR passengers vs going to OOC and changing is about 30 minutes, but that varies depending on which airport terminal the LHR station is built under. Given that there are, and will continue to be, 3 terminal complexes, HS2 passengers for two of the terminal complexes would have to change, and the saving would thus be reduced to around 15 minutes (and effectively more given the frequency of inter- terminal transport)
Complete guess, but it would be in the region of an extra £5bn cost; need to build a tunnelled 4 platform station under the Heathrow complex, with appropriate interchange links to the terminals - that would be £4bn on its own. And it would be around 6 miles longer, all in tunnel, which would be another £1bn at least.
To summarise then; significantly lower benefit, both in financial and sociology-economic terms; net worse for passengers; significantly higher cost.
Hence it is not being done.
Or more simply:
During the analysis, options for a direct HS2 route through the airport were rejected on grounds of cost and the delay to passengers not travelling to Heathrow.
Section 6.11:
https://assets.publishing.service.g...velopment_of_the_HS2_Proposed_Scheme_v1.6.pdf
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