When a journey begins away from a road, we 'snap' to the nearest node on the road network, while applying a time penalty for doing so.
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The maximum 'snapping distance' is 1km, and the time penalty applied is the time it would take to walk the required snapping distance (at the default walking speed of 1.4 m/s).
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Roads where walking is not allowed (e.g a motorway) cannot be snapped to, as a driving journey could not begin there. This means that a route can't begin on a road where walking is not permitted, unless there is an alternative node on the road network that can be snapped to where walking is allowed.