[The object of art is] to make eternal the desperately fleeting moment. – Tennessee Williams
The trick with any arc of headlights is to minimise the full-on glare of the lights, something likely to occur at some point in your frame. Here in the second frame from a moonlight sequence you see a short space between the edge of the frame and where the car lights begin – a gap which would otherwise be occupied by a strong glare.
In general a shutter of 15 to 30 seconds best suits headlight arcs, but it depends a lot on how distant your view is, the speed of the car and how far it travels across your frame. At any rate, your exposure can continue past that if you need to relieve the following darkness. On the other hand, the background setting should only be semi-dark so that the headlight trails show well, as they lose their power in near-daylight effects with moonlight photography. This image has been darkened in post-pro for that reason.
The steep slope in front of the camera is not obvious in the wide angle, but it required a dab hand with the tripod set-up. In such situations four-stage tripods must really come into their own. When fully extended the four stages give you more height than you need – except on steep banks and perches. On slopes the fourth extension puts the camera back up to eye-level… for a line-up you’d otherwise be struggling to obtain.
However I do not own such luxury myself! Another tripod feature which I am increasingly using, though, is the spirit level. Slow learner that I am, I find shining the torch on the spirit bubble beforehand saves time in post-pro with the Rotate button. Although it is no big deal to level a horizon in post-pro, it involves sacrificing a little of the frame, and usually I’d rather not.
28mm, ISO 2000. 15 seconds at f5.6. Standard picture control.