[rytis]

Not sure. Let's say a person spends 15 secs washing, and 15 secs drying. Combined it's still 30 secs. So if we have a long queue, does it really matter that two people are in the bathroom at any given time, or just one? The total processing time does not change.

Or am I missing something here?

[beala]

It actually does help, and the concept behind it is called pipelining [1].

Suppose there's a sink that washes and dries, and there are two people who want to use it. The second person will spend 30 seconds waiting, and 30 seconds washing/drying. So, 60 sec total.

If there's a sink and a separate dryer, the second person will spend 15 sec waiting, 15 sec washing, and 15 sec drying. So, 45 sec total. The second person gets a 15 seconds savings, because he can start washing as soon as the first person finishes washing, rather than having to "block" until the first person is done washing and drying.

[1] http://en.wikipedia.org/wiki/Pipeline_(computing)#Concept_an...

[pbhjpbhj]

But your pipeline stalls as you've people waiting to use the drier, those people block sinks, flow is reduced.

1 sink and 1 dryer: first person leaves after 30s, after 60s 3 people have been served.

2 sink and dryer combos: 2 people leaves after 30s, after 60s 4 people have been served.

I think the analog is that parallel processing is better than pipelining.

[cmsmith]

I think your assumption of equal time for each task is incorrect. Either because people don't dry their hands, or it doesn't take as long as washing.

Because typical bathrooms have 4 times as many sinks as they have hand driers, and I still see sink lines more often than drier lines.

[roc]

Only because "air drying" is a more acceptable option than "not washing". So a combined station would only slow things down inasmuch as people no longer skip drying.

In general (at least in the US) drying takes far longer than washing. Because most people wash quickly/poorly whereas driers are on timers and most are horrible.

And when your hands are still wet even after negotiating the unresponsive sensor and waiting an entire drying duration, it's understandable that many people would just as soon skip it.

[rytis]

OK, but still that doesn't change the equation. 22 secs washing and 8 secs drying is still 30 secs in total, whether it's done separately or not. And as you mentioned, the number of sinks and the number of dryers are balanced, so they combined have a similar throughput. I think you see sink lines more often just for that reason, if the sink line can handle 10 persons per minute, and the supply is 20ppm you will have a queue. But once we throttle people at the sinks, the supply to the driers is 10ppm, thus no lines (assuming the driers line is 10ppm)....