Apologies for the newbie question but I looked for 15 minutes and couldn’t find the answer.
I’m using OSM to generate routable maps for my Garmin and it’s working fine so far. But for some reason I’m not able to specify that a street (way) is a two way street. I’m using both JOSM as well as the online Potlatch editor and in both applications I’m only able to specify streets as one way.
You don’t have to. A road is two-way unless you define it to be oneway=yes
ah, i see the oneway=yes option, thanks!
why is it then that both the editors have arrows showing “direction of way” for the [default] two way streets?
That’s the direction of the data structure you have drawn. That always needs a starting and an ending node, and thus has a direction.
Only IF you specify oneway=yes, that direction is used to know in what direction the oneway heads. Also, some elements like waterways (rivers, etc.) always need a direction (water always flows in the same direction).
That’s a good question… 
Having a more “draw a map” like editor would be usefull, rather than create OSM datastructures. This is one of the reasons I prefer potlatch.