The river decides your route

It is worth putting the whole subject in proportion. A great deal of effort goes into choosing between crossings and it is second-order compared with the simpler variable, which nobody can adjust from the driving seat and almost anybody can adjust from a diary.

Almost every river crossing in London is fine at half past ten in the morning and difficult at half past five. Where a journey time can be moved by thirty minutes, moving it is worth more than any amount of clever routing.

That is advice a client can act on and it is worth giving. A meeting moved from five to four thirty saves more than a driver ever can.

The clearest practical illustration of everything above is the question of which airport somebody uses, which appears to have nothing to do with the river and is decided by it more than by anything else.

From anywhere south of the river, Gatwick is the natural airport and Heathrow involves a crossing or the M25. That reverses for north London. It is the clearest practical example of the river shaping a decision that has nothing obvious to do with a river.

Which is really the point. In London the water is not scenery; it is infrastructure with a limited number of gaps in it.

Most of the central crossings were built for a city with a fraction of the vehicles it now has, and several of them have been closed for structural repair within recent memory. When one goes out of service the effect is felt for miles in both directions, because the traffic it carried has nowhere obvious to go.

That fragility is worth holding in mind when planning anything with a fixed time on the far bank. A crossing that has been reliable for years is still a single structure with a maintenance schedule.