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It's pretty useful to have this tied to the actual ride; otherwise you're bound to forget it and leave it on, or off.

Also, you want some velocity information and/or history, so that someone can see that you've stopped moving and for how long.

Check out Glympse.

That requires allowing Google to retain your geolocation data.

Is Strava a more reliable partner in keeping this data safe than Google?

The iPhone Google Maps app has the feature, although I haven’t turned it on so can’t say how well it works.