A 30x speed improvement


Going from 8 minutes down to 15 seconds. Not bad. What’s the secret?

Synchronizing the traffic lights. 

I’m talking about my evening commute home. There are a set of 4 traffic lights I hit to get out of my Microsoft building and onto the closest highway (520 westbound).  They used to be out of sync such that you’d get green lights when the traffic in front of you was already backed up; and red lights when it was wide open.  (I think in those cases, the lights were optimized expecting a different  traffic pattern then what actually happens)

Generally, if you have a bunch of traffic lights in a short section of road, you want them all green at the same time so that traffic can flow through all of them without getting stuck in between lights and causing backups.

Before they were out of sync, and I’d be sitting at the same light through multiple red / green cycles. Now they’re “fixed”, and I can literally get through all 4 of them in one clean motion.

 

Applications to software:
– contention can kill performance.
– optimizing for the wrong scenario (in this case, wrong traffic pattern) can kill performance.

Comments (5)

  1. Mark Miller says:

    Which way do you leave the garage?  I would like to bask in the glory of 15 seconds as well.

  2. Peter Ritchie says:

    Where we live they synchronize traffic lights to be green when you arrive at one from the other when going the speed limit–not all at the same time.  So, if you make it through one green you should make it through all subsequent lights if you can maintain the speed limit.

  3. Mihailik says:

    This story has a lot of irony in it, since there was a time Bill Gates in his early years created a software for controlling traffic lights. I don’t remember what was the city.

    What’s a shame you still have such a trouble so many years after, isn’t it?

    Sadly, you cannot employ those same programmer again.

  4. Mihailik says:

    >> Where we live they synchronize traffic lights to be green when you arrive at one from the other

    Well, it cannot work for 2-way roads, can it? Unless you desynchronise opposite-way lights.

  5. Mark –

    I go out from behind p4:

    http://local.live.com/?v=2&cid=3E4D8E4B0E394433!765&encType=1

    The pertinent part of that is 600 yards.

    The astute observer probably realizes that given the numbers, 15 seconds is lowballing.

    – My car clock doesn’t show seconds, so measurements under 1 min are inprecise.

    – zipping through all green lights "feels" fast.

    I think these can both be rolled into another lesson about gettin accurate measurements.