Oyster PAYG/Capping Query

As I understand it, if your zone 1-4 cap was at the peak rate (£9.50) and you travel to zone 6 off peak then the fare from zone 4-6 would apply (£1.10) to cover the additional journey as that is the cheapest valid way to calculate the fare for the journeys you will have made.

But if this trip to zone 6 means the balance of journeys you made makes an off peak zone 1-6 fare plus a separate charge for the peak journeys cheaper, then that will instead apply. Although that is pretty difficult to achieve!

Say you made two zone 1-4 journeys during the morning peak (2 x £2.80 = £5.60). You then repeat the same again after the peak period has ended (2 x £2.20 = £4.40). The peak cap (£9.50) will apply as the cheapest fare.

You then make two off peak return journeys from zone 4-6 (4 x £1.10). The off peak zone 1-6 cap plus two peak zone 1-4 journeys (£7 + £5.60 = £12.60) works out cheaper than the originally charged zone 1-4 peak cap plus the four off-peak zone 4-6 journeys (£9.50 + £4.40 = £13.90) and so the cap being applied will change.

That is my understanding of how the system works, anyway.

Just been thinking through what cetacean has said, and understand the logic of it... wondering if it is what is done.

One issue is that as Z1-2 + Z2-6 caps where one is peak & one off-peak < peak Z1-6 cap , final charge for day is dependant on actual journeys made, as well as zones travelled in (and you can save by buying 2 separate ODTCs).

Also the fact that
(Z1-4 peak cap) < ((Z1-2 peak cap) + (Z1-6 off peak cap)) < (Z1-6 peak cap)
and such has 'interesting' implications for his approach.

Which leads to an interesting query - if I make early (peak) journeys in Zones 1 & 2 (to hit its cap of £6.70); and then make additonal (off peak) journeys out to Zone 4 to hit another cap - what should (giving TfL's capping promises/statements - this is the 'right' answer that Oyster should come up with) I end up paying for the day - £6.70 (given that the OP Z1-4 cap at £5.80 is less) or £9.50 (Peak Z1-4 cap)?

The conundrum comes from loose wording in the TfL fares and tickets information (See here ): "Daily price capping is the most you will be charged a day when you use Oyster to pay as you go on bus, Tube..." - and

Peak travel

A peak cap will apply if you travel on bus, Tube, tram, DLR, London Overground and some National Rail services between:

* Monday-Friday 04:30-09:29

If the total cost of your journeys is less than the peak cap, you will be charged separately for any journeys taken during peak hours, plus the off-peak cap.

Off-peak travel

An off-peak cap will apply if you travel between:

* Monday-Friday from 09:30-04:29 (the following day)
* Saturdays, Sundays and public holidays from 04:30-04:29 (the following day)

- and how the various caps inter-relate.

While logically thinking £9.50 is what I should pay (and expect Oyster to charge), I'm not convinced that what TfL has written doesn't lead to £6.70.

The Oyster system has very limited data rate and very limited time/resources to do calculations. Downloading a few bytes of totals and doing some basic arithmetic seems more likely to me than downloading the whole day's journey history and futzing about working out possible steps for different combinations of tickets. Remember capping is implemented entirely on the card/reader. The central system never needs to know the internal workings of capping.

But it never explicitly tells you "we charged you this cap for these journeys, and this much for these other journeys". So I don't think that's what they do at all.

Here's the step by step of how I think it works:
1.Work out the zones used and whether peak/off-peak etc, and what the full single fare would be.
2. Make a list of all caps that completely cover the journey (so a Z1-4 off-peak journey is part of the the Z1-4 peak and off-peak caps, as well as the Z1-6 peak and off-peak caps, and so on)
3. Download the current totals for those caps from the card (or altenraitvely download the journey history and recalculate them)
4. See if any of those caps are at or near their limit. If so, reduce the fare charged to the amount needed to reach the cap (or to zero). If not, charge the full fare. This is the actual fare charged.
5. Increment all of those caps by that charge and write them back to the card
6. Deduct the fare to the account balance (and update the journey history etc)

None of these steps require complex computation or large amounts of data exchange. We've all seen step number (4) happen, which is the most direct evidence I have that this is the implementation.

Err...yes.....I think.....

It's on pages 237-239 (of 239!!). A lot there: I'm a degree scientist but I can't find the bit to cover the original question of what the cap is when there are journeys both in and out of the peak. If it's there, it's mighty well hidden.

It actually starts on page G-2 (204 of 237) - there are several bits on capping (amongst all the other stuff that's going on!).... I think I've made head and tail of it all, but aren't convinced (there's a comment that a worked example would have been useful!!)

One thing that is apparent is that internally the system uses three capping levels (peak, off-peak and all-day - which increase as you use more zones).
Quite what the relation is (or should be) between these I'm not sure, also whether (not necessarily justified) assumptions have been made.

In particular what should be/is the result of making a zone 1-2 journey during the peak (£2.20), and a lot of zones 2-6 (or intermediate) journeys off-peak (but not going into/through z1)? The applicable off-peak cap should be £4.60, to give a total of £6.80.

But it doesn't! All caps apply at all times. There are just some you haven't hit yet.