Vote to ADOPT open for safe duration fields, flexible trip time estimates!

[Weston Shippy]

The vote to adopt has been opened for this PR: https://github.com/google/transit/pull/598

This proposal adds the ability to define the travel duration of flexible ("on-demand") services, relative to direct driving durations that consumers are expected to estimate themselves.

Voting period shall end April 16th at 23:59:59 UTC. 80% in favor results in the vote passing, per the new change process.