In principle, I favor the implementation of a mixed use bicycling/walking path on Chappaquiddick. 
No
I was once the principal planner for the Town of Dennis Bikeways. Mixed use bi-directional paths are dangerous unless they are very wide, have smooth pavement, good drainage, and are free of debris - especially sand. Di-directional mixed use paths generally, create a high liklihood that pedestrians, especially mature and elderly strollers will be injured by inexperienced bike riders. Also, pedestrians bring leashed dogs, kiddie-strollers, baby carriages, toddlers and youngsters with trainning wheels on to a mixed use path which often soon renders it unsuitable for bicycle riders and walkers alike. A mixed use path is generally ill advised. 
Safest would be to get two narrow one-way bikepaths, one on each side of the roadway, marked for travel in the same directon as traffic flow. The realtively narrow one way paths diminishes bike-bike, bike-car, bike- pedestrian accidents and provide for much safer conditions for all involved. Plus, casual walkers are far less apt to use the road-shoulder bike paths to lollygag and dillydally. 
One consistant MV problem is clearing sand from bikepaths - sand makes the path unsafe nad dangerous. Unless we have a budget for labor & equipment to sweep the sand off - this could create a big safety problem. 
Victor Colantonio