@cjepson that sounds amazing…sadly we’re going in June and probably won’t be hitting the ferry till the afternoon - there better not be any mist at that time of the day 
It was a little like this painting.
Sorry, didn’t notice this before. You’ll have to wait for tdn to get some specific answer. The signs will make it pretty clear though. My impression is on a busy day on the Cape all the roads are slow and crowded.
Since you’re on the Cape anyway you might consider taking the ferry out to Nantucket for the day.
Let me add if you’re driving out to LI on a summer weekend you will run into traffic, guaranteed. After you get off the LIE at the last exit it’s another hour on a single lane road to the ferry… past some nice wineries and farmland and possibly the world’s best pies (Briermere Farms). Just plan for it, time wise.
We have reached the end of my expertise on this subject. But my sole memory of 151 is that it seems like it wouldn’t get too busy.
Or MV. The ferry is a lot cheaper, and it’s faster.