The Larz Anderson Bridge, which spans the Charles River between North Harvard Street, Allston and JFK Street, Cambridge, will be rebuilt and reconfigured at a cost of $20 million. Construction will this spring and end in the summer of 2014. If all goes to plan, the chaos of cars, runners, rollerbladers, bicycles and other travelers will be tamed by reducing the current four lanes to three, expanding sidewalks, adding a bike lane and other improvements.