Transport for Lisbon, Portugal
Data about buses routes, parking and traffic
TfL API.
Transport for London, England API is a free Transportation API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.
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Details from api.tfl.gov.uk.

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Data about buses routes, parking and traffic
Data about positions of Metro vehicles in real time and travel their routes
The Transport for London, England API is provided by Transport for London - API (api.tfl.gov.uk). Discover TFL APIs
Yes. The Transport for London, England API is listed in the Transportation category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The Transport for London, England API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the Transport for London, England API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Transport for London, England API is served over HTTPS, so it's safe to use in production.