12 March 2006
SETI League
PriUPS Project


Save Gas With Your GPS

Along with many other Toyota Prius owners, I have become a gas-saving weenie.  I drive at the speed limit, I don't use my brakes, I don't rush to the next red traffic light.  In fact there are a bunch of subtle and not so subtle tricks I and anyone can use to increase mileage.  Most of them are equally valid for ordinary (non-hybrid) gas guzzling vehicles.  In many cases, they are more valid, since if you have a Prius you're already saving about half on your gas bill.  Here's one I invented:  I've never seen it advertised elsewhere, and it will go a long way to paying your subscription to this blog.  Brought to you with the cooperation of the CATWOKE* folks and the PriUPS website, I offer a valuable gas-saving notion.

The First Light is the Killer

Try an experiment:  Get your car up to 55 mph and, when able safely to do so, take your foot off the gas, put your eye on the odometer, and coast down to 15 or so mph.   How far did you coast?  Half a mile?  Three quarters?  If you get 20mpg and have to stop for a light instead of letting the car's inertia carrying you forward, you've just reduced your mileage for that gallon by 2 to 4 per cent!  Are traffic lights evil or what?  Of course, you rarely have to come to a screeching halt for a light.  When you commute, you pretty much know their timing and sequence along your route and modulate your acceleration so as to minimize braking.  Of course you do!

But what about the first light?  When you get off the highway you have no idea where that light is in its sequence, and sometimes - not often - but sometimes, you do have to come to a screeching halt. 

Not any more!

Your GPS Knows Where - and When - You Are

A well-known but only modestly appreciated feature of GPS units is their timekeeping precision.  This is actually a fundamental characteristic of the system:  without nanosecond time precision, metre**-level geographic positioning would be impossible.  The point here is that your GPS will give you accurate time, not to the minute, but to the second (or nanosecond, if you need it).

A well-known but largely ignored feature of traffic lights is that they run on timers or, sometimes, computer control.  Their controllers, in turn, get their timing from the electric grid, which is precise to the microsecond  and whose long-term accuracy is as close to "perfect" as can be arranged.

Put these two facts together and realize that you can look at your GPS and know when the light is going to change.  With this foreknowledge, you can modulate your speed a mile or two before you get to the light so as to arrive before it turns red or after it turns green.  You will never have to stop for that light again!

Some Details

Some of you are way ahead of me, and have already figured out how to calculate the light schedules.  If you haven't, it's pretty simple.  Unless you want to stop and watch, which is sort of cheating, just drive normally and, using your GPS, log the times the light turns red.  After you've accumulated a handful of times, jot them down and look for a pattern.  For example, my traffic light nemesis happened to turn red every minute and a half when there were cars waiting in the cross direction, and would turn green when the sensor decided they had all left the intersection.    The actual times were 00:00 (right on the hour), 01:30, 03:00, etc.  All I had to do was either speed up a tiny bit or slow down somewhat more and I could make sure I didn't get to the light in the first 30 seconds or so after these times. 


But Richard - this is mathematics!  I can't do math in my head!

Yes you can.  If you are reading this, you are probably a human being.  That's one of the things they can do.  (If you are not a human being, please be sure to see www.ieti.org ).

But Richard - I don't have a GPS!

Go get one.  If you're a taxpayer, your share of the $10 billion and counting invested in the system is already up there in space.  A cheap GPS, even if you don't need one for navigation, is a useful thing to have.  (And see follow-up, below.)

But Richard - this is ridiculous and you're being a jerk worrying about such minutiae.

I always have a problem with compound statements like that.  No, it's not ridiculous, and yes, perhaps I am being a jerk, certainly not for the first or last time.  However, this trick works, it saves gas, and if you already have a GPS it will cost you nothing.  Moreover, according to recent medical findings, the brain exercise may save you from premature mental decay with its attendant healthcare costs.

I love the idea, but my traffic lights have no discernable pattern.

Your traffic light is a spawn of the devil and there is no simple remedy.

*Citizens Against the Waste of Kinetic Energy
**I spell the distance of one metre, a little bit more than a "yard," as metre, even though my computer puts a little red line under it.  A "meter" is a device used to make measurements, e.g., a voltmeter.  A metre, little red line or not, is a distance in my blog.

Follow-up 28 July 2006

If you have a Prius with the navigation system, you do have a GPS.  Although none of the standard navigation screens will give you GPS time to the second, see this blogitem for an inconvenient way of finding it.

Follow-up 03 Sept. 2009

Although this blogitem was written before the iPhone was introduced, it suggests a perfect application for it and its conspecifics, such as the BlackBerry Storm.  With the built-in GPS and the user's ability to write or download apps, it could "self-train" to locate traffic lights just from speed patterns and calculate their cycles.  After a few weeks, it might be able to suggest appropriate speeds and display them during your commute.

Richard Factor