From OpenStreetMap Wiki
- Start with two crossing ways with an intersecting node.
- Split each way at the intersecting node if it is not yet split.
- Select the intersecting node. Make sure to press Enter once you've selected it, so that Potlatch doesn't go into drawing mode.
- Click the 'Restrictions' tab in the tag panel, then 'Add new turn restriction'.
- Using the drop-down menus, select the type of restriction, and the 'from' and 'to' roads. For example, if the restriction is no left turn from High Street to Market Street, select 'no left turn', 'From: High Street', and 'To: Market Street'. You can click 'Show' to highlight each road.
- Save your edit to the OSM data base with a helpful changeset comment.
Thanks to Richard Weait, Richard and gumpa for their discussion on OSM help (CC-BY-SA).