Totally, regeneration type stuff is quick to do, such as resurfacing
As soon as you need public consultation, detailed engineering etc the timescales go way out.
Crossrail however is a logistical nightmare. I mean its harder to think of a worse place to undertake such a level of construction than London.
Its built up, its got history so issue to do with that.
But your right, unless the projects ready, or almost they wont get shovels in the ground for years potentially.
Heathrow 3rd runway is the sort of thing, you can make the decision, but you want do much for years. Although lots of consultants will start to make some money quickly