Ditto about the lasers. They can reflect at unpredictable angles from chrome or reflective paint and guess who ends up in court?
Cheap ultrasonic sensors are good to about 4.5 meters and give distance. For a 2 lane road you could set 1 on each side but not across from each other.
A bright light on one side with a light sensor behind an aperture or in a tube to limit the sense angle would work as a beam interrupt if it is a 1 lane road.
Air hoses are what gas stations have long used to signal someone entering the pump area. Hoses could be placed from either side of a 2 lane road but again, each set of wheels would act as a trigger.
How about noise detectors in parabolic reflectors aimed down at where tire noise would be the main source?
If you can dig the road a bit and measure during day when the sky is bright then try rows of light sensors set in the road. Not just 1 per lane though or a car dripping oil or other fluid (radiator overflow, tranny fluid) might take that out.
You might be able using conductive paint and cable, to make a capacitive sensor per lane. I can't imagine that a car wouldn't show up though calibrating out falses might take some discovery as well as work.