STEP ONE - Preparing the straights

Begin by making a 60x60 square. Make a mark at both sides about 20 pixels down from the top. Both marks must be exactly the same number of pixels from the top. If 20, then both must be 20. If 19, then both must be 19. Draw a line connecting the marks. Eventually, this line will become the sides of your road (so don't go too haywire with it).

Make a copy of your square and rotate it 180 degrees.

Make a rectangle that is 60x120. Copy and paste the two squares you just made onto it. Arrange them so they fit perfectly on the new rectangle. It is important that they fit perfectly. The quality of the texture set depends on this part. To ensure proper placement, you might want to make your rectangle a different color from your squares so you can see misalignments easily. You should now have an image that looks like this (with a line 20 pixels from the top and another line 20 pixels from the bottom).

Use the selection tool to select the middle part. Now fill it with a color that will not bleed into anything else. Winterkill used gray so I'll follow his lead.

Now make a copy of this image and rotate it 90 degrees (left or right doesn't matter).

Save these two images.