It's hard for me to tell what the most likely interpretation is, but yes, a comma could certainly disambiguate it. As it was written, I was expecting it to go "

It's hard for me to tell what the most likely interpretation is, but yes, a comma could certainly disambiguate it. As it was written, I was expecting it to go "But since 2000 heavy trucks have accounted for 40% of the TOTAL growth in oil demand, and since [foo] is also true, we expect that [blah]..." but then my mental model just crashed.