Date: Wed, 19 Feb 92 16:24:08 CST
From: firstname.lastname@example.org (Alan L Varney)
Subject: Re: Cellular Calls From Airplanes on the Ground
Organization: AT&T Network Systems
In article <email@example.com> our esteemed Moderator writes:
> [Moderator's Note: I *have* used my cell phone from the observatory at
> Sears Tower. The local expressway/traffic monitors for Chicago have
> their office in the Sears observatory. They sit in a large room with
> plate glass windows so that the public may peer in and see the various
> hookups they have to radio stations using their services, etc.
The stuff you see at Sears Tower are remote "monitors" connected to
the Illinois Department of Transportation's Traffic Control Center
(TCC), which has the computers driving the displays and providing all
the radio station hook-ups, etc. TCC is located in Oak Park,
overlooking the beautiful Eisenhower Expressway, with four PDP-11/70s
monitoring the 2200 traffic sensors and 90+ entrance sensors buried
beneath the Chicago Expressways.
The PDPs tabulate the sensor data and forward it to a large VAX for
actual processing, including algorithms that estimate the travel times
over various Expressway sections. A recent IEEE-sponsored tour of TCC
was very enlightening. The data collection equipment looks like a
small telephone office (lots of wires, etc.) and their problems are
similar (outside plant failures, etc.). Sensor data is transmitted by
home-made Frequency Division units that stack 23(?) sensors onto
leased telephone lines.
TCC says they operate the most modern traffic monitoring system in
the world, from humble beginnings in 1960. They believe their
reports, coupled with rapid accident removal (the IDOT "Minutemen"),
allow the expressways to carry 20% more traffic than un-monitored
systems, and reduce travel times by allowing "flexible" travelers to
choose another time or route. The entrance lights that control the
entry rate in many areas serve to keep (where possible) the average
traffic at about 25% density, which seems to yield minimum travel
times with maximum throughput -- more cars actually reduce the total
number of cars traveling a given distance within a given time. TCC
also runs digitized voice information through low-power transmitters
(at the far ends of the AM band) located at prime intersections of the
Note that IDOT is not currently allowed to monitor area tollways;
they're operated by another state agency.
I can provide more if desired ...
Al Varney - AT&T Network Systems, Lisle, IL
Date: Mon, 24 Feb 92 23:57:28 CST
From: firstname.lastname@example.org (Alan L Varney)
Subject: Chicago Traffic Monitoring
Organization: AT&T Network Systems
I'll try to keep it short, interesting and telephony-related ... two
errors in the previous article; it's the Traffic System Center (TSC),
not the TCC, and they use PDP 11/53s -- not PDP 11/70s.
This information is from a paper presented to the Regional
Conference on Traffic Management and Planning for Freeway Emergencies
and Special Events, Nov. 1991, Toronto, Ontario, Canada and from the
July/Aug. 1991 issue of "Home and Away" (Ill.-Ind. version),
published by AAA-Chicago Motor Club.
Chicago motorists frequently hear reports such as:
"On the inbound Edens, it's 50 minutes from Lake-Cook, 25 from
Dempster. On the Kennedy, it's 55 minutes from O'Hare , 22
from the junction, and 15 in the express lanes ..."
Data for such reports come from a network of about 2000 sensors
under the 130 miles of Chicago-land Expressways (but not Tollways),
collected and processed at the Traffic Systems Center in Oak Park, IL.
The TSC supplies information to various radio/TV stations, etc. where
it is made available to hundreds of thousands listeners each day.
The Illinois Department of Transportation (IDOT) operates TSC as
part of a program begun in 1961. Computerization of traffic
information in the early 1970s was a cooperative effort of TSC, the
AAA-Chicago Motor Club and WBBM NewsRadio (AM 780, a CBS affiliate).
Reporting on WBBM is sometimes from Chicago Motor Club staff at the
Sear Tower's 103rd floor "skydeck", which connects to TSC via two
terminals plus other displays. WGN Radio (AM 720) has a graphical
display at the Tribune tower "showcase" driven directly by TSC's main
Is giving the travel times more effective than just indicating
"congestion" on various Expressways? A survey at the 1990 Auto Show
suggests that up to 85% of those questioned had changed their travel
plans or routes because of this data.
Raw data for the reports comes from sensors buried in each lane at
about 2000 foot intervals. The sensors consist of loops of wire under
the pavement, usually three turns in a three-foot square. Vehicles
passing over will cause changes in the loop inductance. The presence
of a vehicle is reported by sending a tone at a frequency assigned to
the individual sensor over leased telephone lines or IDOT cables. (I
think 23 frequencies can be "stacked" on a single line/cable pair.)
These lines are monitored at TSC by individual units tuned to a
particular frequency for each sensor. Each "line" is apparently
connected to multiple units. This technology is basically unchanged
since the early 1970s, and reminds me of early T1/DS1 plug-in units
and frames. Four PDP 11/53s each sample about 25% of the units,
collecting samples up to 60(?) times/second/sensor. This data
(vehicle present/absent) is reported to a VAX 6210 over Ethernet(?).
The VAX is mid-sized (32 MB memory, two RA82 622 MB disks), connected
to six DECservers. These provide the processed reports to terminals,
graphical displays, etc.
It appears that data is smoothed over 30 second periods, and most
reports are at five minute intervals. Interactive terminals can
request "raw data" -- the current data for ten or so points updated at
five second intervals. The more processed reports give travel times
every five minutes, derived from lane occupancy sensor data. I'm not
a Transportation Engineer, but they seem to use "%-occupancy" as well
as estimates of vehicle speed to determine the travel time.
One of the interesting charts shown on our tour was %-occupancy vs.
"throughput" of an expressway. At less than 20% occupancy, adding
more cars increases throughput and has no effect on speed. From about
20 to 35%, adding cars increases throughput but slowly decreases
speed. Beyond 35%, adding cars decreased both speed and throughput --
near 100%, the traffic is stalled and speed/throughput approach zero.
TSC provides the reports (at no cost for the data, just the cost of
the connections) directly to several IDOT locations, four State Police
Districts, the O'Hare Parking Center, METRA (Commuter Rail) and PACE
(Suburban Bus) headquarters, and about 15 radio/TV stations. And the
"Shadow Traffic" and "Metro Traffic" services provide these reports to
50+ other media outlets.
IDOT operates TSC as part of a three-part freeway traffic management
program. The other parts are:
(1) the Emergency Traffic Patrol trucks (called "Minutemen") have
operated since 1961 to render quick assistance for traffic incidents.
These 35 trucks provide emergency gas (two gallons/$5 billed later),
water, air, tools and off-freeway towing. All but the gas is free.
The trucks actively patrol the freeways, to spot hazards and minimize
response time. In 1990, over 100,000 assists were provided, mostly
for disabled vehicles. Main location is at 3501 S. Normal Avenue,
(2) the Commuications Center (IDOT District 1 headquarters at 201 W.
Center Court, Shaumburg, IL, 23 miles from TSC. This center handles
all radio dispatching, hot lines, traffic/maintenance coordination,
incident management, interface with police, etc. They receive all
citizen reports of traffic problems, including 10,500+ calls each
month via Cellular *999 calls. They also run the 10 low-power AM
radio broadcast stations (at many major interchanges on AM 1610), with
digitized voice driven directly from the TSC reports. Versions of
this are also available on Cellular *123, "menu #6 (?)", then select
the proper expressway.
312-245-1132 provides same information, tone phone needed.
708-705-4618 provides travel advisories and congestion information.
312-DOT-INFO (via menu #6) provides access to the (708-705-4618)
information, and other menu items provide access to other IDOT
Experimental delivery mechanisms under test include alpha-numeric
"pager" delivery and a cable TV graphical display with audio times.
TSC has several other responsibilities. They were first with ramp
"metering" signals; there are now 95 such ramps. Signal timing is
varied automatically (with a manual over-ride available) based on the
"%-occupancy" calculations. Each such ramp has sensors for the first
two spaces before the signal, and one after. It is easy to observe if
anyone is not waiting for a "green" light, or is "piggy-backing" on a
previous vehicle's light.
TSC also provides a clearing-house for emergency travel information
distribution for State Police, METRA, PACE, O'Hare Parking, etc. Any
media connection receives this information along with the five minute
The expressway "changeable sign" system is also controlled by TSC.
Some 13 signs at key positions can be used to give road condition and
emergency information information to drivers that are not listening to
"traffic reports" on the radio.
"Studies have shown the TSC plus other measures have reduced peak
congestion by up to 60% and accidents by 18%. A trucking-industry
study suggests that $17 in benefits are returned to the public for
each $1 spent by the system."
Al Varney - AT&T Network Systems
Disclaimer: I have no affiliation with IDOT except as a periodic
highway user and taxpayer.