No, you should treat people like adults, set a deadline, and let them execute to that deadline. Many people balance multiple tasks, objectives, and goals and so taking 10 hours for a task that was assigned 10 hours is perfectly reasonable. If you keep planning 10 hour car rides and they continuously come in at 5 hours, you're bad at estimating.
So you're ultimately saying people should be given 5 hours for the car trip, even though giving them 10 would only result in a ~6-6.5 hour trip with healthy bladder relief?
No, I’m saying if you have 10 hours to do something that can be done in 5, but is optimally done in 6.5, take 6.5 hours and use the remaining 3.5 to do something useful. Maybe you’ll go on a sightseeing tour at your destination; maybe you’ll resolve a customer-reported bug you didn’t think you had time for this release.
If I have 10 hours to make a car trip, but it only takes 5, I don’t drive at half the speed limit. Sure, there may be some extra bathroom and snack breaks, but probably not 5 hours worth.
I see people underperforming due to low expectations quite often, possibly following the classic 80/20 rule. I think this “planning against the deadline” behavior is one of the difference-makers between a hungry startup that outperforms, and the legacy player that barely keeps up despite having 10x the staff.