I am also facing same problem for tunnel lighting.
Again I think you might be calculating incorrectly according to whatever standard you're trying to conform with.
Traditionally tunnels will have only gentle curves, and typically have very close luminaire spacings, meaning that the effect of the curve is negligible.
For most street lighting you don't concern yourself with the upward/downward slope (grade) that most roads exhibit, nor with the camber of the road surface (the curvature from middle down to sides). You're not asking about the change in the 'height' of the road surface (which is natural, since you're probably mostly looking at a plan of the road, and not the side elevation/section of it).
I think if you investigate how you're supposed to comply with the standard involved, you'll find that you do not need to take the curvature of the tunnel into account.