The only time I drafted a big truck was 30 years ago on a trip to Florida.
It got cold at night, I did not have good gear, and I drafted the truck for an hour to keep warm.
Cars and vans I can see through I draft as I have an under powered bike.
Not real close, but closer then is 100% safe.
If I can not see ahead, I do not do it.
When traffic is heavy and fast, the entire hiway gets a big draft going and adds 10 mph to my bike.