Where is the Meridian highway, which is mentioned in Chapter 15, about mid-chapter, in respect of Courthouse Square, in To Kill a Mockingbird?
The east-west highway, called Meridian because it connects Maycomb with Meridian, from where Dill comes each summer, is about a mile south of the Square beyond the main residences, including Scout's and Boo's, and beyond--to the south of--the school property.
If we look at some of the detail of Maycomb's layout, we see that to the east of Courthouse Square is Atticus's office. Across from it, on the west side of the courthouse, is a road that one map draws and says leads to Barker's Eddy (misspelled as "Baker's Eddy"). Running to the south from the front of the Courthouse is the main residential street, terminating in the cluster of houses for Mrs. Dubois, Mr. Avery, Miss Maudie, Miss Rachel and Miss Stephanie and the Atticus Finch family. A bit more to the south, along Second Street, is Deer Pasture, the Boo Radley house (and the tree with the knothole) and the school houses and yard. Meridian Highway runs east-west along the southern perimeter of the school property. On the far side of Meridian Highway (at the south edge) extends a northerly-southerly dirt road that Scout narrates as leading the way to Barker's Eddy (since some maps put a road to Barker's Eddy to the east of the Courthouse, it's possible that that country road is thought to be long and to wind around down south to Barker's Eddy).
I was wide awake, remembering what Dill had told me. Barker’s Eddy is at the end of a dirt road off the Meridian highway about a mile from town. It is easy to catch a ride down the highway on a cotton wagon or from a passing motorist, and the short walk to the creek is easy,....
So, with this configuration of locations in mind, and with Scout's narration in mind, we can say that, in respect of Courthouse Square, the Meridian highway, leading to Dill's hometown of Meridian, is about one mile south of Courthouse Square, lying beyond the Finch and Radley houses and lying at the southern perimeter of the school property.
In Chapter 15 of To Kill a Mockingbird, Scout narrates that the swimming hole, Barker's Eddy, is located
at the end of a dirt road off the Meridian highway about a mile from town.
From this we know that the Meridian highway is about one mile south from Courthouse Square.
Since the courthouses of old Southern towns are located at the center of the town, the distance to the Meridian Highway would not differ much from the description of the distance to any edge of town, with Courthouse Square as the central point.
One could suppose that in a small town such as Maycomb in the 1930s, the edge of town and the location of the courthouse on the town square would often, if not always, be less than five miles.
On today's maps Alabama Highway 21 runs through Monroeville--which is fictionalized as Maycomb in Harper Lee's narrative--only 230 feet from the town. From Route 21 one goes to U.S. 84W leading to the connecting routes to Mississippi. Today it is still 138 miles from Monroeville (Scout's Maycomb) to Meridian on the Meridian highway.