I think it is a really bad decision.
It changes nothing, And indeed makes many bad things worse.
Imagine if you will trying to build an airport the size of Gatwick next to Heathrow? How do you get the passengers in and out on the already overloaded M25, M4 and M3? How do you find space for the car parking? Where are the cars to go? What about the support industries that develop all round an airport? That is in effect what will happen.
On top of that an extra runway at Heathrow does not allow for 24 hour operations.. I know 24 hour is unpopular, but the world id global and people come to London from all over the world. More aircraft at Heathslow will not make access to the rest of the country easier.
A more efficient option would be a second runway at Stansted.. (And I do live near Stansted!!) with a new road West towards Milton Keynes and a third crossing into Kent somewhere near Southend.
Ready for thee incoming.