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.
I think what Daniel is referring to is in IESNA RP-08 and in TAC there is the requirement to place calculation points along the quarter width lines of a particular lane (essentially the tire tracks or position of the driver and passenger's perspective as they travel down the roadway). The calculation grid creation tools in AGi don't make this easy if the lanes are curving and you want to generate an illuminance calculation. If there was an option to mimic the roadway metric tool used to create luminance calculations (2 clicks along the alignment and 1 click for the width) but allow it to create an illuminance grid and multiple selections (multiple clicks along the alignment and 1 click for the width) then it would be appreciated. The work around like Daniel mentioned is to generate a polyline to give a long continuous path of the lane and use the array option to get your other quarter line or multiples if on a multi lane road.
I have seen many poor designs with AGi where someone has treated the road like a parking lot or intersection and just defined a perimeter area of the edge of pavement and applied a X by X grid pattern to fill it's limits. This does calculate the illuminance of the area but when we are looking at a roadway and following RP-08 and TAC this is not what they depict in their examples. I have even seen some where drawings were submitted for review and the illuminance grid limits are up on the front lawns in a residential area! As if they are trying to illuminate the entire right of way to the recommended levels.