That other site is mostly full of morons, so take whatever they say with a grain of salt and then some. I traveled down the highway today for a factory visit in Horana, and the experience was over all too quickly, since I had to get off at Galanigama exit. But even then, I shaved off at least an hour of travel time (usually would have taken the Galle road instead), and a full 25kms 15kms_each way_. I only spent 10 minutes on the highway, travelling close to 120, but eased off to a hair under 100 when I saw the fuzz (twice). Coming home was even more pleasant, having left at 5:00pm, I was at home by 7:30. And that was after having stopped at the Kottawa Pizza Hut to dine in, and them having mucked up my order.
The lack of the frustrating galle road traffic, numb right leg and extreme urge to beat a cyclist's head in with his own helmet is fully worth the 300 bucks I spent at the toll booths.
P.S. I forgot to add that every single driver I saw on the road adhered to the rules, driving in the left hand lane only, even the vans. The only exception was a 2011 red 6 series convertible that whizzed past at roughly around 140-150.