Not a dumb Brit question...these things can vary. Not sure what the culvert looks like at Bowser, yet (haven't checked it out nor read all these past few pages), but I'll post a couple photos I took 10/14 a few miles away of the culvert that runs under the road on Spring Valley just east of Brick Row Dr. It's large enough to walk under.
ETA:
Culvert at E Spring Valley just east of Brick Row Dr (looking south towards Spring Valley):
Drainage pipe dumping into side wall near this culvert:
Spring Valley Rd above this particular culvert (I crossed the street to peek over the other side):
Another 'under-the-road' culvert (Hillcrest Rd at Valley View Park):
Again, the one at Spring Valley/Boswer Rd where LE made their discovery may look different than these I visited last week, but this is what is typically meant by an under the road culvert. Hope this helps.