...tapers the yellow lines at both ends...
Not sure about photoshop, but on gimp this is easy - you just click the 'Fade Out in X pixels' box in your paint tool, enter half the desired length of the line as X, and draw each line from the centre out to the ends. I assume photoshop has something similar in your brush settings, something like 'simulate brush pressure' or 'fade'.

Alternatively you should be able to apply a gradiant to your line (ie a line that starts off white than turns into black). If so, you can draw directly onto a black layer mask masking your required line colour, starting each stroke at the centre of the desired line. That should have the same effect with only a little more work.