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February 28, 2007

Congestion Pricing, Positioning, and Meshed Wireless Networks

As part of my internship at the Regional Plan Association I was asked to research the applicability of mesh networks to congestion pricing for New York City. What follows is the result of several days of reading, surfing the web, talking on the phone, and stroking my chin. It assumes some knowledge on the topic, most of which can be found in descriptions of London's Congestion Charge, upon which any scheme in New York is likely to be based.

Primary Questions

  • What about London's CC scheme do we not like?
  1. Pricing is not very flexible. No variability of charge over time or space (i.e. path)
  2. For the most part, only charged for crossing the boundary into the zone
  3. Post-payment (i.e. account-based billing) is impossible
  • What would we do the same in a first implementation?
  1. Charge people for driving within a certain area during a certain time period
  2. Use cameras to charge people who opt out of any other system, i.e. cheaters and tourists
  • What would we want to do differently, ?
  1. Charge people with accounts, like EZ Pass
  2. Charge people who do not cross the zone-charging boundary (i.e. remain entirely within the zone)
  • What would it take, from a technologic perspective, to do it as we prefer?
  1. Substantially higher accuracy of detection
  2. Detection within the zone, not just at its edges
  • What technologies and approaches are likely candidates to be considered?
  1. Automatic Number Plate Recognition (ANPR, cameras reading license plates, like london) for enforcement
  2. Dedicated Short Range Communications (DSRC, like EZ Pass) for point detection for account-holders
  3. GPS positioning for area-wide detection
  4. Wireless Positioning System (WPS -- TV/WiFi/GSM) for area-wide detection
  5. Wifi/Mesh Networks for communications
  • What sort of physical footprint or envelope, both in the vehicle and on the streets, would we expect for each different solution?

Continue reading "Congestion Pricing, Positioning, and Meshed Wireless Networks" »

April 12, 2007

Urban GPS is Now

Generally speaking, the Global Position System does a great job of letting you know where you're at. A well known problem in urban areas results from tall buildings which occlude satellite signals as well as reflecting them, causing so-called multi-path errors. A good example of how these problems preclude the use of GPS for certain urban applications can be found in Transport For London's "Technology Trials." The image below shows the size of the buffer zones that would be necessary to calculate with 99% certainty that a car with GPS entered London's Congestion Charging Zone:

(source: Transport for London)

Enter, Skymeter. To enable their GPS-enabled parking and congestion charging business, they have developed algorithms for correcting GPS signals in urban canyons, as shown here (white is the position from a standard GPS chip, yellow is the post-processed SkyMeter position):

(source: SkyMeter Corp)

Toronto? Do they even have tall buildings there? Well, SkyMeter recently went to London to test their system in the same areas of London that caused TfL's GPS vendors so many problems. The results are similarly impressive (red = standard GPS, green = SkyMeter)*:

Zooming in a bit:

(check out the cool PostGIS, GeoServer, and Google Maps-powered interactive version)

To get a sense of what they were up against, take a look at TfL's calculations of the number of GPS satellites available in central London (red = less than 4, yellow = 4 to 10, green = more than 10, all in the 99% confidence interval):

(source: Transport for London)

Even after the Galileo (the euro-GPS) goes online, the test area is still no better off, according to TfL's models:

(source: Transport for London)

I think the data speak for themselves. Skymeter is currently planning a number of pilot installations including one for parking in the City of Winnipeg, and I wonder what London and Stockholm, or other cities contemplating congestion pricing (eg New York), will do with it. How much it really changes the discussion of what technologies are suitable for congestion pricing right now I'm not so sure.




* A disclaimer from SkyMeter, who was generous enough to give me their data to make those London maps:

This data is the "before" (red) and "after" (green) of the data processed by Skymeter with its first alpha test of data collected in London, UK, in heavy "urban canyon". This same data was analyzed by a third party (Mapflow from Ireland) in London. They calculated error as point to line perpendicular distance, and reported a reduction in the 90, 95 and 99 percent error quantiles of 26% 34% and 48% respectively (i.e. in meters). While this says the Skymeter process clearly removes error, this metric does not measure the second to second error variance, in otherwords, Skymeter removes absolute error AND makes the process much better behaved. This provides [1] a far-stronger evidentiary record, [2] two to three orders of magnitude greater compression, [3] spatial error bounding and [4] extremely rapid pricing-map registration (a trivial form of map matching at the data center)

This process is the first of four parts that [1] de-noises the GPS positioning signals, [2] characterizes the residual error for non-refutability, [3] bounds the spatial error, and [4] bound potential financial error. This process is part of a patented process that Skymeter claims can reduce tolling errors for GNSS-based tolling for distance-based road user charging as well as GNSS-based parking metering to arbitrary levels -- for example to one bill in 10,000, or to an arbitrary percentage such as 0.01% of a bill -- including in urban canyon.

July 18, 2007

Zero Sum Game (in a good way)

[err, so now NY may get it together on congestion pricing, but the train of thought is relevant nonetheless]

While usually calling something a zero-sum game is a bad thing, in this case I mean it positively. The $500M for a congestion pricing pilot that New York has lost may still lose would go to somewhere else like Dallas, San Diego, Atlanta, San Francisco, Denver, Miami, Seattle or Minneapolis. I am a New Yorker born and bred, and am as disappointed as anyone about this, but I think in this case we all may be suffering from a slight case of New-York-is-the-center-of-the-world-itis.

None of these other cities have nearly the mass transit system that New York has. Probably combined their mass transit systems don’t carry half the passengers ours does, and most people living in those places think of cars as an absolute necessity for practically everything they do. In a broader sense, it could very well be an overall net positive that the congestion pricing pilots happen in other cities. Perhaps those places will make incremental progress in shifting people out of cars and more importantly, changing peoples' value systems when it comes to cars vs. other modes.

If we are really lucky, this could be the first wave in a national shift towards more rational thinking about transportation, which would definitely benefit NYC in the long term. In truth, I'm a lot less worried about NYC than I am about other cities and the country/world as a whole, so I wonder if it doesn't hurt to at least imagine the possibility of some greater good coming from Albany's (seeming) ineptitude.

December 3, 2007

Congestion Pricing is a Technology, Remember?

In many ways London's system for Congestion Pricing should be model for New York, but in other ways it really isn't. The most obvious way that it isn't is in the actual technology proposed to do the job. Yes there are cameras and computers involved, that's sort of where the similarity ends.

Specifically, in the UK tradition, all of the cameras relay a full video feed to some central processing location. Not only is this absurdly costly (think fibre!!) but it allows for plenty of privacy invasion by anyone who has access to the cameras' feeds. The proposition for New York is very different. The proposition is much cheaper and seems to all but eliminate the possibility of using the cameras for anything but looking at license plates. That's because the cameras would be equipped with enough smarts to know when to snap a photo, and only that still image would be sent to be processed. If you don't believe me, read this excerpt from IBM's recently released proposal:

A worst case analysis shows that for a very busy lane, with one thousand vehicles passing the detection equipment every hour and forced to send two 100kB images for each vehicle, the bandwidth requirement is a mere 57kB/s. This is within the capacity of wireless networks today, but is not the optimal solution approach.

A more realistic case, in which 50% of vehicles are equipped with an E-ZPass tag, 90% of the remaining license plates are read with a sufficient confidence at roadside and 80% of charges are paid in a timely manner, leads to a bandwidth requirement of 8kB/s. A very busy, six-lane detection point would thus be well within the capacity of NYCWiN, even without local reinforcement of the wireless network.

We estimate that, with our proposed solution approach to vehicle detection at the edge of the network and given the estimated amount of traffic in the city, the average local bandwidth requirement across the system will be on the order of less than 1kB/s per lane, and the overall load on the backbone of the wireless network will be small.

More generally speaking, I think it suffices to say that Congestion Pricing uses technology, and as we know technology only gets better and cheaper over time, so we can be sure that NYC's Congestion Pricing technology will be much better and cheaper than London's.

Now, if only somebody would only explain this to all the privacy freaks and civil libertarians that are making this process so painful...

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