I don't think there is really a standard.
I guess it depends upon what type of roads you want/like.
Standard road lanes today are about 10' wide, and I use a scale of 1.5mms per foot for 1/144th scale, so that would be only 15mm wide, or about 5/8ths of an inch.
For paved, modern roads, I was a little more generous, and went with 1" (25mm) wide lanes, and an extra 4' for a shoulder, so 1.25" wide each way, or 2.5" total width, including the shoulders. These are meant to be nice, main roads, through the European countryside.
Without shoulders, or in more rural areas, they could be narrower.
I'm also using 1" wide lanes for a planned autobahn section, and with the 1/4" (4 foot) shoulders on the sides.
Obviously, dirt roads, and/or gravel roads and tracks will probably be more narrow, so perhaps about 1/2" – 3/4" wide for those.
Rivers and stream widths, as needed/desired. Some people like to factor in the game scale with these too, but if you do that in some cases, they'll just look silly, so you need to balance them a bit with the miniatures size as well.
Hope that helps.