I make this commute from NJ to Williamsburg regularly. Best route depends on traffic conditions which relate to the time of day. I cross the GWB around 6:30 AM. If I am on the Southside walkway on the bridge, I make the first right onto Cabrini Bvld (one block against traffic), a right on to 177 St and an immediate left onto Haven Ave. Follow Haven to the traffic signal at Fort Washington Ave and make a right turn. There is some traffic through Columbia Presbyterian area. Follow Fort Washington Ave to Broadway. Make a right on Broadway and an immediate right on 158th down the hill through the traffic light to the stop sign. There is a handicapped ramp on the right hand side of the intersection that will take you to the Greenway, or follow the ramp down (more traffice) to the left for access. Follow the path to Horatio street. Cross 9A there and follow Horatio (one block with cobblestones) to Greenwich Ave. Make a 45 degree right on Greenwich and cross 6 Ave where Greenwich becomes W 8 St. Follow W 8 (aka St Marks Pl) to Ave A. Make a right on Ave A to left on Stanton to right on Suffolk and then onto the WBB.

Crossing Lower Manhattan this way before 7:15 AM is actually very quick. After 7:30 AM traffic gets much tougher. An alternate here is Bleeker which has a path much of the way. But I find myself stopping for more traffic lights.

I usually depart Williamsburg around 3:30PM. Across the WBB. I turn right on Suffolk (1 block against traffic to avoid Clinton). Left on Rivington, right on Norfolk, left on Houston, right on Ave A, left on E 3 st, right on Lagaurdia Pl, right on Washington Sq S, left on Washingto Sq E, left on Washington Sq N which becomes Waverly Pl across 6th Ave, first right turn after 6th Ave and an immediate left onto Stonewall Pl which turns into Christopher St. At 7 Ave bear slightly right onto W 4 st, 2 blocks to Charles St. Left on Charles St down to the path on the river. Up to the handicapped ramp and back up 158 St to the second left turn, (first and second left turns are close to each other and both are labeled Riverside Dr), take one of the next streets to the right and follow 1 block to Fort Washington Ave. Left on Fort Washington to left on 177, to right on Cabrini and onto GWB.

There are many variations, but I have found this the easiest. If the North walkway is open on the GWB I will use the pedestrian bridge near the end of 181 St.