As someone who drove his motorcycle today instead of taking the train, I'm getting a kick out of this. Costco runs on the train are quite difficult. About 4 times a year, I go to Costco and the extra storage on my bike means I can make those trips at that frequency rather than more often. Most of my other shopping is done on foot or, if I need something from a specific shop, by train. Some things (like large blocks of cheese) I can only really find at Costco here in Japan. Same with American-style bacon.
Speed limits are definitely lower here, and our trains are better than Amtrack, so I think both of those number will be lower.
The idea of specifically choosing motorcycle as your Costco mode of transport is hilarious, but I suppose panniers beat lugging a bunch of bags on a train. Also, my brother did a foreign exchange program in Okinawa when he was in high school, and I'm often reminded of how he said his host-father pronounced Costco as "Co-su-tu-co".
I don't own a car and renting one for the day is stupidly expensive. I'd also have to drive a car on Tokyo streets and I'm not a fan of that at all, heh. With a bike, at least it's not as claustrophobic.