SOUTH PORTLAND — It’s the busiest stretch of the busiest street in the city, causing commuter headaches for years and leading some to question whether a plan to consolidate two aging middle schools on one site might worsen traffic congestion. The road is Broadway, a winding, mostly two-lane, cross-city artery that’s nearly unavoidable if you want to get from one side of South Portland to the other without driving far afield. The worst section is a 1,000-foot, four-lane bottleneck between Evans and Lincoln streets that carries about 24,240 vehicles daily, according to the Maine Department of Transportation.