This was studied, and it was determined that 13th/14th and 16th/17th are actually the best place for cars, and Colfax for transit.  Your logic is backwards in terms of what helps/hurts local businesses.  People in their automobiles generally don't even see many businesses as they fly by, and cannot stop quickly to patronize a business they pass.  People on transit, or on foot (transit and pedestrian travel are joined at the hip) by contrast DO generate business for local shops - at a significantly higher rate.  

Colfax has also been determined to not be a very efficient automobile route anyway because of stoplights at many, many intersections.  Most people with any sense are already using 13th/14th and 16th/17th because you can travel MUCH faster on those streets than Colfax.  Colfax is a destination, not a freeway, and slowing down the cars in favor of transit and pedestrians will HELP the businesses, not hurt them.