ok,I would like to know what would be the best way to make the lines in the concret to represent the tarmack?I'm using evergreen sheet styrene as my base for the tarmack.
So I was thinking about scribing the lines but how would someone scribe the lines/squares that were 15mm X 30mm? Also,another thing,the base is 12.5in X 10.5in.

Mark the edges at 1.5 cm intervals on one edge, and at 3 cm intervals on the perpendicular edge. With your ruler, join the marks from edge to edge to form your grid, then scribe along the grid lines.

You may wish your grid lines to not be parallel to the edges of your base. In that case, draw a line at the angle you want your gridlines to be, then draw a second line at 90 degrees to it, so it crosses in about the middle of your scene. Make the 1.5 cm and 3 cm marks as above, then construct your grid.