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).

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. Mouse over road with avenue selected: Same as above Mouse over avenue with avenue selected: Same as above

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. The gridline displayed for a given road depends on what road you moused over and what road type you currently have selected. Turning on the 'gridlines' feature when plopping a road tile will display gridlines for the last two road tiles you mouse over. Luckily the game itself provides an excellent tool to help you with this. 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. To answer this I'm going to need to establish some criteria for 'ideal'.

Simcity 5