SimCity 4/Roads

A city in SimCity 4 relies heavily upon its road system to get commuters from A to B. Because of this, designing a city's road system is critical to the city's success. All zoned lots (residental, commercial, industrial) require road access, and many civic buildings will also benefit being placed next to a road. Two types of roads are available for use: main roads and streets.

Roads
Main roads are black tar-sealed with a yellow centre line. These heavy-duty roads can support a good number of cars without becoming clogged, and should be the main type of road used when constructing a road system. The slightly higher maintenance cost should not dissuade a city planner from using roads over streets, as they are superior in every other way. Be aware that although roads have a high vehicle capacity, in sufficiently large cities, they can still become clogged by an excessive volume of traffic.

Roads can utilise bridges, tunnels, highway on-ramps and neighbour connections as required. Bridges and tunnels are constructed by dragging the road builder tool over a body of water or sufficiently steep grade, respectively.

Streets
Streets are grey roads with no centre line. Akin to backstreets, these minor roads can support less cars (presumably due to narrower lanes and a lower speed limit), and their use should be restricted to side streets and similar uses where the number of cars travelling on the road is expected to be low.

Streets cannot use bridges or tunnels, cannot connect to highway on-ramps and cannot connect to neighbouring regions, but they can be automatically placed during zoning; this behaviour can be suppressed by holding the Shift key whilst using the zoning tool.