Got back today from two days' camping and riding at Allaire. Just wanted to say thank you to everyone, as a result of the info I got here, I had the maximum amount of fun (and minimal amount of pain) possible.
First, the campground itself was good. I don't camp a lot so I don't really have much of a basis for comparison, but, site #23 was close to the restroom/showers, private and well kept. The restroom/showers were basically clean but the only hot water I could find in the showers was in the wheelchair accessible stall. Worked fine.
Second, the riding/access to Allaire was -exactly- what I hoped for.
Mitch's instructions were perfect and the 3-4 miles of doubletrack / asphalt / walking paths was a nice warm-up and cool-down. The Hospital Road bit right before the entrance to the parking area (or after if you're leaving) is sketchy as hell though, no shoulder at all. Fortunately it's only a short bit, maybe 2/10ths of a mile.
Third, this was, overall, pretty much what I signed up for. Set up camp, go riding, come back, eat, sleep, repeat. Which leads me to the ROAD NOISE. Holy crap! When I got there (daytime) I could hear the cars but it wasn't bad. Nighttime is another story, as so many people have mentioned. Thanks to this input, I brought my noise-cancelling headphones and made out mostly okay. Fun fact: noise cancelling headphones don't seem to work when the sound is the BEEEEEEP BEEEEEEP BEEEEEEP BEEEEEEP BEEEEEEP of a dump truck backing up. Lucky me, I got highway construction one night! Nice. The second night was better except for a barking dog at 1:30 am, that was super fun.
Finally, the overall vibe of the campground was awesome, it's a small place, people were well behaved, though I had to laugh at the campers next to me literally letting their car run for HOURS so everyone could keep their devices charged. I got a
solar-powered power bank (not an affiliate link) that worked well for me.
I also saw a lot of the rest of the park (non-cycling stuff) I never knew existed. It really is a great park, AND a great place to mountain bike.
Thanks again everyone!