From the southwest, LA 42 starts at an intersection with LA 30 in southern Baton Rouge, Louisiana near Louisiana State University. It has been designated with the street name Burbank Drive. From its start for about 7.6 miles it travels on a mostly easterly path until it makes a northerly turn intersecting Highland Road. At Highland Road, Burbank Drive ends and LA 42 picks up the name Highland Road, LA 42 makes an easterly turn. From there it has several direction curves from east to north where it eventually intersects I-10 (Exit 166). From I-10 it goes in a northeast direction until it intersects US 61 Airline Highway. At Airline Highway LA 42 shares the road with US 61 out of East Baton Rouge Parish and into Ascension Parish in a southeast direction for 3.3 miles. At this point LA 42 turns east and then northeast until it crosses into Livingston Parish and intersects LA 16. For the next 3.1 miles, LA 42 and LA 16 run concurrently, with LA 42 in a northeasterly direction at the end of the concurrency. LA 42 intersects LA 63 in 7.4 miles. About 2.5 miles after LA 63 LA 42 will make an easterly turn, its final major directional shift. It progresses on an easterly path until it ends in Springfield, Louisiana at LA 22.