Anyway we are thinking of taking the GS Parkway to the Cape May ferry and down through Del and MD to the Chesapeake bay bridge tunnel before hooking up with 95 south of DC. We would likely take the 6am ferry and get into Del around 730. Has anyone done this before and if so is it worth going that way instead of down 95 and hitting traffic?
I think you should choose your route based upon month/day/daypart. We use the ferry frequently for travel, so as an experienced ferry traveler, I'll give you my thoughts.
1. New schedules aren't out yet for April and beyond. Check to be sure you can get a ferry that early on the day you want to travel. Even in high season, I think the only ferries before 7 a.m. are on the weekends.
2. Once on the DE side, follow a route for Rt. 24 to Rt. 113 and then to Rt. 13. You will keep moving that way. Going down the Delmarva is a pleasure once you get to Rt. 113. There's even rest areas along the way which are well-kept.
3. There's a rest area just before the Chesapeake Bay Bridge-Tunnel. But, there's also an observation area on the bridge with facilities. It's a bit of a hassle in heavy traffic to get in and out of there, but if traffic is moderate to light, it's a nice place to stop.
4. After the bridge, strap yourself in and make sure you are wide awake! The tangle of roads around the VA Beach/Norfolk area and their accompanyng road signs, even combined with GPS directions, are somewhat insane if you're actually going past the area. You will see nearly NO signage for anyone traveling past to Rt. 58 and over to Rt. 95.
THIS is a reasonable objection to taking the ferry-route, because it has taken us several trips to do it the way we intended.

So, my advice is to memorize,
memorize memorize your route numbers and intended direction on each segment, and pray if you are so inclined.
5. The drive on Rt. 58 to Rt. 95 is easy, about 1 hour.
Generally, I find that we're getting used to doing things one of two ways when using the ferry and heading out for FL.
1. Take the last ferry and drive overnight. I stay in the car and nap on the ferry while the family goes up on deck for about an hour. I drive most of the night while my kids and dh sleep in the car. DH will drive a couple hours, usually through NC, and go right back to sleep once I'm back behind the wheel. Once daylight breaks, he generally takes over and I snooze. All of us do well in the car though, but we make sure we have a dull week before we leave when we choose this option. So far, we've only done it twice, but we like it.
2. Take a mid-day/afternoon ferry. I get all the last minute running around done, and dh works part of the day. We then travel to the VA Beach/Norfolk area and stay overnight. (It's so much easier to have a destination
in the area we've found, than trying to get past it. Masterful marketing? Maybe!!

) We eat, chill out, shower and sleep. My family is really easy to get going in the early morning, so when I get up, I pack us up (usually 4 a.m.) and we're on the road. Everyone goes back to sleep and I drive us along. We bring breakfast and lunch with us, and it takes us (roughly) 12 hours to get to WDW. So whatever time we pull out of VA Beach area, we know that's what time we'll get to WDW in the afternoon. No grousing from the family if I get us going before 4 a.m., 'cause they know we'll be there that much earlier-->

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DON'T try to take the ferry in the height of the summer traffic and expect to make good time on the GSP through Cape May County. There's 3 lights towards the southern end ("Exits" 11, 10, 9) that will make you wish you had taken your chances with Balt/Wash/Rich traffic!
IF you can get your group going at 2-4 a.m. I'd avoid the ferry (even if you live far downbeach at the shore like I do). At that time of day, it would be an anomoly for you to have any issues on I-95.
So, my answer is, it depends. But, we take the ferry far more times than not, simply because we find it fits the times of day we prefer to get going.