Hi,
Make sure you have enough space between the streets. To actually fill the space in between, you need to close the streets to blocks.
--> just create new thin streets connecting all the ends of those streets.
so, steps :
1] street left
2] street right
3] street middle
4] raise middle street
5] connect left middle right ends with new streets, on both ends
6] > 2 blocks are created between all segments.
7] change block subdivision to 'no subdivision' or an other type
8] 'grow' your structure until you have the full thingy built.
check that you're working with accurate distances and street widths. activate the information display (d,d key) and the navigation display ( d,n ) and check the grid size.
note that in this whole process you are 'misusing' the street system to build the geometries you need.
using the 'c' key enters the 'edit streets tool', this lets you edit the tangents too. also try Graph Menu > Set tangents smooth.
ok ?
Matt