I have noticed this many times and it happened again last night. I parked on the third floor and the way the parking structure was built I had to drive up to the sixth floor in order to be able to go all the way back down to the exit.
Why? My supposition is the cross over between the up and down ramp takes up a few potential parking spaces. But only a few. These are big lots and almost never, ever full. Does it really matter if they can say they have (say) 12 more spaces out of hundreds?
I’ve lived in or near a big city for most of my life and have never encountered what you have described. I’m interested in hearing home common this is in the US, assuming that’s where you live.
To handle the case where everyone is leaving at the same time (e.g. after an event).
Interesting. I had not thought of that. I’m a bit unclear on how making everyone go up before down helps that though but I can maybe kinda-sorta see it? (IANAGarage engineer)
If everyone leaves at the same time, there will be a bottleneck somewhere: the parking garage egress, the first stoplight, a traffic officer, etc.
Making everyone go up, then down creates an artificially long queue to “store” all the cars. It is the same idea as the roped-off queue at the airport security line. It stores all of the people while they go through security.
One of the parking garages where I work was designed for high volume/high turnover (it’s a hospital parking garage intended for patients and visitors, not employees) and the traffic flow is helical up and helical down. In other words the up ramps go to odd numbered levels and the down ramps go to even numbered levels. Because of the high volume, all ramps are one-way and you have to go all the way to the roof to get back down. This is to prevent cars going in opposite directions when people are pulling out. It’s a function of volume - similar to what CaveMike said.
Page 11 of this document has some nice diagrams of the different types of multi-level parking garage ramps and flows:
I think this is a big consideration. If a garage is busy, there would be more delays with bi-directional traffic and more for drivers to look out for.
Some other considerations: Uni-directional traffic also allows slanted parking spots while bi-directional traffic allows smaller lanes because they can be shared (I expect).
Thanks. I’ve always wondered what the set of ramp patterns are and page #11 makes it pretty clear.