I have been able to find good eateries on the highways of Punjab and on the GT Road in Haryana. But my experience on the Highways of Rajasthan and Gujarat have not been as good, though tolerable quality food can occasionally be found if you are lucky and if you check up with threads on BCMT or with people who are familiar. In UP, on the Delhi Meerut road, there are some good locations. Gajraula on the Moradabad road has Giani and a few places. Agra Highway has also many decent places including the high end ones at Mathura. Kerala Highways are pretty good for food joints. East Coast Road from Chennai to Mamallpuram has excellent reestaurants including some in the town. Meddur on the Bangalore Mysore road is famous for some Veg Tiffin Rooms. For the rest of the highways, the general statement that it is difficult to find good food holds true.