Is it possible to calculate luminance when the road is not straight?
I'm not sure where you're located, but in Australia we work on a combination of calculations and design rules for roads.
Curved roads achieve compliance by assuming first that the road is straight, doing a calculation based upon the road width (and of course pole setback, outreach, tilt, etc.), and gaining a spacing figure. That spacing figure is then used to place fixtures along the roadway, combined with design rules that take into account curves, intersections, cul-de-sacs, etc.
This is done for both Category V roads (where the visual requirements of vehicles are dominant) and Category P roads (where the visual requirements of pedestrians are dominant). Category V roads are the only ones where we take luminance into account.
So here, for a Category V road, I'd calculate a straight section, then apply the design rules in relation to curves. The Standard shows that you can use the same spacing as you would for a straight section of road providing the road (or road reserve width - I can't remember without looking at it just at the moment) does not 'break' a direct line of sight from one fixture to the next. The assumption is that if the curve is of a higher radius then it's pretty much like a straight road. If the radius is small (so the curve is tight), you've got to bring the fixtures closer together until the road (or road reserve width) doesn't 'break' that direct line of sight between fixtures. After the curve you go back to your standard spacing and other design rules.
In Australia I'm sure there's plenty of people getting this wrong, and placing calc grids all over the roads in their site plans and claiming compliance due to meeting certain LTPs, but at best it's an approach that doesn't technically comply with how the Standard is written, and at worst it can make it look like a design complies when it is impossible for it to do so.