Great to see you passed. I had to take a pre-employement and a random in the same week once. You never know.
I did run across an interesting rule for people that fall under the US DOT drug testing.
49 CFR 40.193 (a) (2) Urge the employee to drink up to 40 ounces of fluid, distributed reasonably through a period of up to 3 hours, or until the employee has supplied a sufficient urine specimen, whichever occurs first. It is not a refusal of test if the individual declines to drink.

Could be helpful in certain situations. They'll still make you sit there for 3 hours. But if you can't pee they'll have to reschedule.