In SimCity, there are two different types of road. These are streets and avenues.
Streets are one square wide and avenues are two squares wide. All streets are the same width, so a dirt road can be upgraded to a high density street using the upgrade road tool.
In order to upgrade a street to an avenue, however, you need to destroy the old street and replace it with a larger avenue, which will remove all infrastructure that exists along the old street.
If you're in a situation where you already have the maximum capacity version of a particular style of road but don't have the space to replace it with an avenue, there are other options available. It is possible to build additional roads within the vicinity to alleviate the amount of traffic travelling down your currently over-crowded road. To do this, bring up the traffic data layer to see where all of the traffic is coming from, and build new roads that intersect the busiest streets to provide additional routes for your sims to travel down. The traffic data layer can be accessed using the 'All Data Maps' button in the bottom right of the interface, or by toggling the 'upgrade road' option.
In addition, there is the possibility of setting up mass transit. Mass transit works by building a depot (and adding additional trucks as required to increase the number of sims that can travel per day and reduce the wait time for busses) and placing stops. The region that a stop will cover is highlighted along the roads as you place the stops. For the maximum effect you will want to place stops along your busiest streets (see your data layer for traffic again) and ensure that the wait time for passengers is managed to ensure it doesn't get too high.
You can monitor wait time by bringing up the information screen for your mass transit depot, and you can monitor the amount of traffic a particular stop is attracting by bringing up the information screen for each individual stop.
The main benefit in the space is (as you said), it is exactly the right room for density growth.
The other benefit of the last example, is that you control which street those buildings will face. All of the traffic they generate will be there. Emergency vehicles will stop only on that road, avoiding jams on the other roads.
Best Answer
To answer this I'm going to need to establish some criteria for "ideal". I'm going to assume that most people are interested in the most efficient use of a given space for high density buildings allowing you to stuff as much population/commerce/jobs into your city as possible.
Luckily the game itself provides an excellent tool to help you with this. Turning on the "gridlines" feature when plopping a road tile will display gridlines for the last two road tiles you mouse over. The gridline displayed for a given road depends on what road you moused over and what road type you currently have selected.
Mouse over avenue with road selected: The first gridline will provide enough room for two high density buildings back to back making for maximum use of the space given the two road types you want to use.
Mouse over avenue with avenue selected: Same as above
Mouse over road with avenue selected: Same as above
Mouse over road with road selected: The first gridline will provide just enough space for a single high density building to exist between the roads. The second gridline will provide enough space for two high density buildings back to back and a gap inbetween that could fit a road (or is potentially useful for narrow parks).