The Express Lane Isn’t Faster
Reader Mike had sent along this great post from a California math teacher who analyzed supermarket checkout times (data, as pictured above, was provided by the supermarket manager). I’m slow to post this and it has now been around a lot, but this was catnip to me as I love these sort of operational/logistical/queue problems (and this relates a bit to the airport walkway problem), particularly when they seem to exhibit that classic “slower is faster/faster is slower” effect. Not to mention that “other lane is always moving faster” problem that plagues us in traffic is a very real issue in queuing as well (and is partially why some outfits use single lines).
Among the many interesting findings:
The express lane isn’t faster. The manager backed me up on this one. You attract more people holding fewer total items, but as the data shows above, when you add one person to the line, you’re adding 48 extra seconds to the line length (that’s “tender time” added to “other time”) without even considering the items in her cart. Meanwhile, an extra item only costs you an extra 2.8 seconds. Therefore, you’d rather add 17 more items to the line than one extra person! I can’t believe I’m dropping exclamation points in an essay on grocery shopping but that’s how this stuff makes me feel.
There’s ways this can be applied to traffic, but reader Mike was wondering about those express/local lanes on highways. I only know anecdotal stuff here, like stories of engineers changing the estimated times on both segments when they really want people to use one or the other. But this is a bit of a guessing game every time I approach the George Washington Bridge on I-80. I’ve been burned many times by the express lane — is it the very wording, which fools me into thinking it’s a better way to go than that inevitably cluttered and slower “local” lane? It of course depends on many variables, like the intended destination of traffic, etc.
Maybe there’s some geeky studies somewhere of tolling as well; exact change lanes versus others, etc.; though those are likely to be outmoded with EZ-Pass etc.
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This entry was posted on Tuesday, September 22nd, 2009 at 9:36 am and is filed under Traffic Wonkery. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
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September 23rd, 2009 at 12:07 am
Actually, if you dig into his analysis, you’ll find that his statement that “the express lane isn’t faster” cannot be substantiated by the data he uses. He uses a single data set (assumed to represent an express checkout). In order to make that statement he would need a second data set for the regular checkout over the same six-hour shift.
Based on the Customer/Hour figure, each customer spends an average of 1.56 minutes in the express checkout, with half that time spent on coupons and payment. A minute and a half, sounds about right. So if there are five people waiting at the express checkout, add a minimum 7.8 minutes to the wait time.
Until I see data for regular checkouts, I would hold off on reaching any conclusion or any application to other situations.
September 28th, 2009 at 8:54 am
Here is a real road example I frequently encounter:
A signalized intersection on a 55 mph road. I am approaching the queue of vehicles waiting for the green. The right lane has 3 semi trucks and the left lane has 10 cars; both queues are approximately the same length. If my objective is to get through the intersection and up to speed as quickly as possible (once the light turns green), which lane do I choose?
Most drivers opt for the lane with cars, the logic being that cars accelerate more quickly than trucks. I choose the lane with the trucks for the following reasons:
1) I’m not in any particular hurry and I want to maximize my fuel economy.
2) Truck drivers tend to pay attention while car drivers don’t. When that light turns green, I am betting fewer than half of those cars drivers are not watching the intersection and are, instead, talking on the phone, messing with the iPod, eating, and nose-picking. Some are doing all of the above.
More often than not, I choose correctly. Complex systems like the left lane (more actors and more decisions to be made) or the checkout express lane (more actors and more people fumbling to extract credit cards) are much more prone to breakdown, and delay.
September 30th, 2009 at 4:29 pm
Three observations:
1) These queue examples are examples of the Efficient Market Hypothesis. In other words, if one grocery or traffic queue were really consistently faster, people would crowd into it until it wasn’t any more.
2) Bicycle lanes are rarely congested in properly run cities. This mode of transit is so much more efficient that it is easy to respond to congestion by allocating more road space to bikes and away from cars. Thereby eliminating the congestion. There are some examples in New York City, and a gazillion examples in Copenhagen and The Netherlands.
3) To bikermark – In Toronto, express lanes are (usually) cash only.