Here in Texas, toll roads use open-road tolling where you don’t have to stop to toss change into a basket. Instead, you come to an archway over the road and just keep on going. A device in the archway interacts with an RFID chip attached to the windshield of the car and charges your account the appropriate amount. Some roads (mainly in the Austin area) don’t even need the RFID chip. Instead, it takes a photo of the license plate and sends a bill to the vehicle’s registered owner.
The speed limit on portions of SH130 (a toll road near Austin) is 85mph (the highest posted speed limit in the US). Supposing the area in which the toll device can interact with the RFID chip is ten feet long (no idea, just supposing), I am in that area for only eight hundredths of a second. That’s not long.
Any idea just how good these devices are?
(I am not advocating driving at excessive speeds nor using techniques to avoid paying tolls. I suspect that the answer is faster than a typical car can be expected to travel. I’m just wondering if anyone knows.)
One of the early episodes of Top Gear had them beating a speed camera by going past at about 175 MPH. The same principle might apply to the camera toll sensors-- basically the sensor and the camera are at the same point, so after a speeding/non toll paying car is detected there’s a built-in delay to let it get into the frame. If you’re going fast enough to be out the other side of the frame, you’re home free!
I suppose they could have the sensor set forward in the road so it detects the car at the instant the car actually is in frame, but I think the ones I’ve seen anyways don’t.
The technologies are unrelated. The speed camera just takes two (or more) pictures a known time apart from each other and sees how far you moved between frames. The RFID systems send out microwave signals that are passively reflected, and encoded with information in the process, by the in-car device.
Some cameras do take two pictures against the road markings, but that’s usually just an additional line of evidence. They still have a radar or laser gun that is what the camera uses to determine whether or not to take a picture in the first place.
In the Top Gear bit, they were dealing with a camera that was just a radar gun and then took a single picture. As the Stig kept going by at progressively faster speeds, the resulting picture showed the car further and further along in the frame until at 175 it was gone entirely. I imagine completely evading one of the ones that takes two pictures would probably require going even faster, though.
That’d be a new land speed record if you do, though. Just think of the glory! I mean, the old one is only 763 mph, going over 1500 mph would be almost double the speed.
The real answer is faster than the police helicopter.
– Any people want to guess what doing 1500mph/3000kph would be like? Would a collision at that speed mean the utter vaporisation of car and contents, or something more like a huge explosion followed by a shower of metal bits? If a honda civic ramped into a battleship at that speed would it punch a hole in it?
My understanding of Delorean physics is that 88mph is the minimum speed needed for time travel of any sort. The computer seemed to handle the time/date placement if I recall. Happy to be wrong though, it has been an awful long time since I have watched the good professor.
There is actually a project to be the first land-based car to exceed 1,000mph (thus breaking the current LSR of 763mph in the process): Bloodhound SSC. Lots of interesting stuff on that site.
As to the actual physics of this, and the OP, I can’t help I’m afraid. Needless to say, nothing road-legal (or even physically capable of driving on a road) is going to approach these speeds.
Isn’t it a matter of moving out of range of the RFID before the transaction can complete? Although, to avoid a fine, you have to avoid being photographed as well. I wonder which effect will take place first: moving out of frame, or motion blur?
If there’s an arch 20 feet or so over the road, I doubt its effective “read zone” is only 10 feet long. Assuming the read is effective at 15 feet (transponder 5 feet off the ground) then consider a 45-degree angle each way, which gives the distance where signal is half the strength. That’s a stretch of road 30 feet long, 15 feet in each direction. If the signal has to go through metal roofs too it is probably fairly sensitive.
Response times of electronics can be in milliseconds.
Of course, this is self-defeating. They tried the same thing with Highway 407 in Toronto; the system took photos of cars that did not have transponders. When too many people were driving without transponders, the system was overwhelmed by having to process several times more photos than the initial estimate. I assume this is how your system works, that if there is no electronic toll response from the car, the system takes a photo and you get a bill in the mail.
So if you’re just trying to beat the transponder reader, you also have to figure out how to beat the “no pass” camera system too, which brings us back to the Top Gear discussion. At this point, why would you bother with a transponder? If they identify your vehicle, they send you a bill. You respond with “I have a transponder, ABC12345”, and they deduct the toll from that at the office if they are nice. If you try to beat the system some other way (obscure the license or tape over it with a print-out of your boss’s license) if this is your regular commute, after a month or two the state trooper will be sitting there watching for your model of car.
I wasn’t really thinking this would be a cost-effective way to evade a toll. I’m not really advocating evading the toll in the first place. I just wonder how fast one would have to go.
1500mph seems a bit unreasonable. I suspect that important pieces of my car might fall off. :eek: