Here in Anderson Indiana, each lane was about 12 feet wide. It was dependent upon the type of roadway (main street, residential or back country road, etc) but most business streets and highways were built at about 12 feet per lane. Earlier, in the twenties, roads were only about 20 feet wide total, so, if the portion of the town you wish to model was built in that era, a 20 foot roadway is fine.
If each lane were 10 feet wide, that would be 1 and 3/8s inch. So a two lane street would be 2 and 3/4 inches. 10 feet is what I use for the lanes of my city streets. Modern hiway lanes would be wider. Parking lanes would have been more narrow in the 50s, say 8 or 8 1/2 feet.
As a guide in HO:
8' = 1 1/8
10' = 1 3/8
12' = 1 5/8
15' = 2 1/16
20' = 2 3/4
25' = 3 7/16
Around here, little has changed in the city streets since the late 40s. A few sidewalks have been removed to widen the lanes and a few streets have had major work done on them to add additional lanes, but the width still stands at about 10 to 12 feet per lane and most side streets are really about three lanes wide total. Small towns would have been this way as well. Some of the main streets in small towns actually had wider streets and also used angle parking.
Country lanes and county roads were much narrower back then, in some cases no more than 12 or 13 feet total. If you met a farm truck you were in big trouble. Part of the problem with all of this is that the width of streets and roads vary according to where you are and what the local street and hiway departments did. This is why there are no set widths for streets in any scale. You can calculate what you need according to your area and era always keeping in mind what looks good to you. Rather than calculate 3.5 mm equal 1 foot, I just used a scale rule and a regular ruler to change the actual scale feet into inches so the figures are not exact, but are very close. I highly recommend a scale rule. The one I have is a 'General' no. 1251. It's been well worth the money.
Oh, one other thing, sidewalks in the business district were about 10 feet wide.