The city transportation department on Thursday announced it will begin construction on the Bronx’s first-ever “busway” along East Tremont Avenue next month, marking the first step in a push to make the street the borough’s premier thoroughfare for bus riders. The one on Tremont Avenue will run a half-mile in both directions between Third Avenue and Southern Boulevard. “It does get a lot of traffic, you just got to roll with the flow, it's the New York flow.” And the New York flow is slow for Bronx riders. According to city data, the average rush hour speed of the Bx36 bus — which travels across East Tremont Avenue — is a mere 5 mph.