A general question about road repair
I've asked in my town's sub but I thought to ask here. My town fixes roads in two passes, driving lanes and edges. I'm not sure if one is always first, but I see new blacktop roadways, leaving about 1-foot gaps at the edges near the curbs. Later, a separate project rebuilds all the curbs and edges in concrete (and steel forms?) and feathers it to meet the blacktop. Because of scheduling and resources, the gaps stay there a long time and most roads in town have them.
What's the thinking behind using different materials for the middle and the edges, and for having separate project plans to do them at different times?