Transport for Germany
Deutsche Bahn (DB) API
Grenoble public transport.
Transport for Grenoble, France API is a free Transportation API that requires no API key, is only available over HTTP and does not support CORS, so it should be called from a server.
Details from mobilites-m.fr.

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Deutsche Bahn (DB) API
Data about buses routes, parking and traffic
The Transport for Grenoble, France API is provided by MobilitesM (mobilites-m.fr).
Yes. The Transport for Grenoble, France 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.
No. The Transport for Grenoble, France API requires no authentication, so you can start making requests without signing up.
Not directly. The Transport for Grenoble, France API does not allow CORS, so call it from a server, serverless function or proxy instead of front-end JavaScript.
No. The Transport for Grenoble, France API is only available over plain HTTP, which browsers block on HTTPS sites. Call it from a server instead.