Road Hiearchy

With most cities, your roadways are the veins of the city. transportation from suburbs from to the downtown locations require a structure rather than just one type of roads across the entire city or the traffic will likely spike.

Small roads

Put these in more suburban / rural areas of a city. you don't need a 8 lane road in the middle of your suburbs. all of the roads in residental districts of your suburbs can use these.

Medium roads

The 4 lanes could be used well in suburbs for utilities (health, fire, education, etc) as well as low-density commerical.

Large roads

The main veins of your urban areas should use these. they may include 6-lane roads (3 lane two-way or 6 lane one-way). Highly populated districts can use these, but use them sparingly.

Highways

Highways allow access into the city from other locations off the map as well as the core of the cities that lead across the entire city. They take up the most space but you have to be careful with them as rather than lights / stop signs, have proper onramps & offramps so there never is any backups on the highways.