Bending an existing line isn't the way to do that. The way that
I would do it...?
Step 1: Create a new layer, between your tarmac layer and your straight line layer. Using the Elliptical Marquee tool with the precise cursor, click and drag from the bottom-right corner while holding the alt (draw from center) and shift (locks 1:1 proportion) keys. Drag until the marquee reaches the far side of your original line.
Step 2: Fill the marqueed area with color.
Step 3: Repeat the marqueeing process of step 1, this time only drag as far as the near side of the original line.
Step 4: Delete colored area, then delete original line layer.
You could also mess around with the Pen tool using Bezier curves, but that's more maddening than it's worth.
