Hello and welcome to the forum. Thank you for your message.
To clarify: OpenStreetMap derived data will only be contained in Journey Planner responses (whether via the Unified API’s JourneyResults method or the standalone Journey Planner API). Within this data, it would only be present in geographic data (i.e. line strings) and walking and cycling legs. Furthermore, we do not return OpenStreetMap data directly but rather it informs our routing engine. For this reason, it is our understanding that our use of OpenStreetMap would constitute a Produced Work, and many of the conditions of the Open Database License do not apply.
If you would like to tell us which TfL open data you are using, we could let you know whether this may contain external data.
There are several reasons for us using OpenStreetMap. For instance, the data is of a very high quality, it is a lot more granular than our current dataset, and it is available free of charge. It has a lot of data on cycle routes and attributes for roads, which will allow us to provide better walking and cycling journeys. It is also constantly being updated, meaning we can benefit from the most up-to-date information, and if there is an error, we can correct it ourselves. We have previously contributed data to OpenStreetMap and intend to do more of this in the future; in this way we and other data consumers can obtain data from a single source, avoiding the problems of trying to combine multiple geographic datasets.
Given these and other factors, we do not believe another dataset exists that is comparable. As @Thibaultmol mentioned, OpenStreetMap is used by several transport authorities in the UK and abroad; this also gave us confidence that this is a tried-and-tested dataset that we can rely upon.
I hope this helps. Please let me know if you have any further questions.