If I’m listening to the radio, and the traffic correspondent says, “Currently it’s 55 minutes from downtown to the airport,” what do they base that on?
Is there an algorithm that they develop for the main arteries taking into account density and speeds at certain points? Do they “eyeball” it from the helicopter based on their experience? Does someone actually do the drive and tell them?
Or is it none of the above? I’m curious.
No need for an algorithm. The easiest method is to figure out how fast the traffic is moving and multiply by the distance to the landmark (the Loop to O’Hare, let’s say 20 miles, traffic moving at 20-25 mph, voila! 55 minutes.)
By the way, I assume you’re from Chicago. I don’t hear estimated times in traffic reports for Kansas City and St. Louis, the two other places I’m familiar with. It may be because traffic in those cities is much more go-like-hell-then-stop, and less a slow, steady stream like I usually see in Chicago.
In San Antonio, they post those time estimates on big signs over the freeways. It’ll list the next couple of interchanges or big exits and list the approximate drive time to each one.
I don’t know how accurate they want to be, however. Last time I was in San Antonio, I saw those, but managed to beat the time by around 25% to each interchange. Maybe they post an overly pessimistic time estimate, so that when you get there early, you won’t gripe about how bad the traffic is. Instead, you say “Ahhhh, I beat the TransGuide time! Traffic isn’t so bad after all.”
Happy motoring.
Why yes, I am in Chicago, kunilou. I thought it might be an average, but, especially on the trip to O’hare, driving speed in traffic will vary considerably throughout the trip. 5MPH for a few miles, 35 MPH for a mile, standstill for a few minutes, etc. I guess a side question would be, how accurate are those estimates…
While Kunilou’s explanation makes perfect sense I still don’t see how this works.
Does a helicopter pick one car far below and time its travel between two landmarks? If so then Stoli’s observation still holds. I live in Chicago as well and having to drive from downtown Chicago to Naperville (about 35 miles) everyday I’m painfully aware of the vagaries of traffic in Chicago. I’ll sit still for 30 seconds, move forward a 100 feet at 10 mph, sit for 5 seconds, race 1/2 a mile at 35 mph then stop again. The helicopters must pick distant points to get a nice average or else they could easily get misleading results. I hate to be the guy (or gal) who has to eyeball one car in the thousands and keep track of it from 1,500 feet up. Worse, the car you pick migh get off the expressway before passing you second point and you have to start again.
As to how accurate these estimates are (at least in Chicago) they seem to be pretty good. The usual drive time given by the radio announcers from O’Hare to the merge is about 40 minutes (during rush-4 hours). I generally make this trip in 35-45 minutes. I’ve never found them to be off by more than 10 minutes.
Living and driving in Chicago, I have also often wondered from where they got the travel times. Having heard traffic reports in numerous other cities (philly, boston, DC, new york, etc.) most big cities list traffic by the location of the start of the bog down, and let you figure out how long its going to take you. I started to think it was a cool idea to list the travel times, until I started realzing they were bubkus.
[WAG]
I have an hypothesis on what the travel times mean in Chicago. It is apparently the time it took someone who has recently arrived at that destination. For instance, “45 minutes from the Loop to Lake-Cook” means that someone who is arriving at Lake Cook Road was passing the Circle Interchange 45 minutes prior. My reason for saying this is these are NEVER the travel times for me at the point I get on the road. I in Little Italy right near the Circle, and if it says “60 minutes Downtown to O’Hare” and it’s 8:00 AM, I know it will take me at least an hour and a half to get to the airport. If I hear the same thing at 10:00 AM, I usually can get to the airport in half an hour. (Jeff’s personal experience aside)
[/WAG]
Notice I said the ‘merge’ which is about halfway between O’hare and downtown which is still in keeping with your time estimates. I agree that 60 minutes from downtown Chicago to O’Hare at 8 a.m. can only happen on Christmas day if Christmas happens to also fall on a Sunday ;).
In the DC area, advertisers mistakenly equate miles to minutes, assuming everyone can drive 60 mph to their client’s business. “Just 4 minutes off the Capital Beltway” means 4 miles from I-495, but doesn’t mention that this is only at 2:00 AM.
In Chicago there are sensors in the road (similar to the ones at intersections that control traffic lights). All that information is fed into a computer that calculates the travel times.
From the Gary-Chicago-Milwaukee Corridor Transportation Information Center FAQ:
Where does the data come from?
The system receives raw data from the Illinois DOT Traffic Systems Center (TSC) and MONITOR once every minute. The TSC receives its data from loop detectors embedded in the pavement on the expressways. The loop detectors act like metal detectors and can sense when a vehicle is near them. This allows the TSC to count the number of vehicles which have passed over each detector (volume) as well as how long each detector was occupied (occupancy). Simple formulas have been developed to convert this data into travel times and congestion estimates.
stolichnaya, why don’t you call the radio station & ask, then get back to us with the answer…
Yeah, because Ghod forbid that anyone should come to the Straight Dope seeking to eradicate ignorance or to General Questions hoping for an answer… :rolleyes:
Traffic reporting at most stations is not actually done by an employee of that station. More likely it is done by a company that provides traffic reports for several stations in a market (Metro Traffic and Shadow Traffic are two big ones that operate in several cities). They make their money by selling those 10-second ads that the traffic reporter reads at the end of the report.