For most of your banking to make the hairpin (behind the start line), you used the same sized ramp (one meter), except at the open ends where you dropped the sizes down in steps(0.75, 0.50, 0.25). This meant you actually made large steps, which if any vehicles hit, would result in same damage a real vehicle would have hitting a large kerb or set of steps.
All I've done, is made every ramp the same angle, and continued them off the track so there's no hard edge, and where I wanted a smoother turn, just used a smaller width (eg 2m instead of 4, 8 or 16).
At the inside of the far hairpin, I just continued the ramps at each edge of the semi-circle you had, so that there were no hard steps at the start and end to give a smooth transition from flat to ramp.
For some objects, you can overlay them without an error, but for other objects that give you an "can't add: intersecting object" error, just increase/decrease the Z height, then when you've placed the object in the spot you want, just change the Z height back.
That's what I had to do with the tyres at the base of the start lights. It's what I also had to do with the red and white barriers on the inside and outside of the large hairpin, to give a smoother curve.
To make your layouts better, don't put lights, signs, etc, on the race track. This isn't done in real life. Put them out of the way, and signs can be put on gantries, lamp posts, other road signs, fences, buildings, etc. You can easy make your own.
Also, on race tracks that have distance countdown markers on one side of the track, it's always on the opposite side of which way the corner goes. Eg, for a right hander, signs are on the left, and vice versa. And as you made the layout, you turn up to a bend or corner, you know which way to turn, but why should anyone who hasn't driven it know which direction to go?
Layout making is just practice. Try to keep straight lines smooth (don't just plonk armco and walls down close to each other and out of alignment), and try keep curves smooth.