26 February 2022

What do you think is happening here?

If anybody is not familiar with interstate highway I-16 in Georgia, it connects Savannah to Macon and pretty much goes like a straight line east-west. There are virtually no towns or cities in between. As a result the 120 mile or so highway has a consistent speed limit of 70 mph.

Sharmila and I were coming back from Hilton Head on this highway. Somewhere in that highway, I noticed something interesting. The display on the rental car dashboard informed me that the speed limit was 40 mph. (see the top middle portion of the picture) I was not in a construction zone and the highway was totally clear.

It took me some time and experimentation to understand what was going on.

Can you guess why the car was giving me completely wrong info (suggesting 40mph in a clearly 70mph stretch)?

If you want a slight hint, check out the other displays on the dashboard in the picture.

Posted February 26, 2022 by Rajib Roy in category "Puzzles


  1. By Rajib Roy (Post author) on

    Somya Chaudhary, Tim Alexander, Nanette Arceneaux and Utsav Kapoor got to the answer or was close enough.

    The car was misreading the Minimum Speed Limit signs (40 mph) as maximum speed limit. What is funny is that it would realize that something was amiss and after a couple of minutes it would switch itself off. Next time a 70mph sign came, it would light up as 70 and then stay there till it got fooled by another 40 mph sign!

    What was the clue on the display?
    It is that small sign over “7” in that 12:57. That is an icon to show that your lane assist mode is on. If you do not see it, it is off. It is that irritating feature in cars that makes the steering wheel vibrate or makes noise if you cross a lane line without putting your indicator on. (it thinks you are not paying attention and drifting)

    In the old days, the speed limit on the car display worked differently. In my 15 year old car, the GPS database in the car has the knowledge of speed limits of the roads. I have never upgraded it. The problem of course is that speed limits change. My car unfortunately always shows the old speed limits – which is wrong.

    Today, cars are equipped with cameras and often Lidars. (I think Lidar is the one that tells the car in front is too close – so your car prompts you to brake in some of the newer cars). Lidar cannot help in drifting across lanes. The (optical) cameras “see” if you are crossing the lanes.

    Turns out the same cameras try to read the road signs. And of course, the bugs have not been all worked out. I tested this in a few ways. For example, I tried a double lane crossing on I-75 when we came to a sign. Since the cameras were pointed away from the road somewhat, it completely missed the sign. In fact, if I was even two lanes over driving straight, it would not pick up the signs. But moment I came to the nearest lane, it started picking up the signs.

    How it realized that something was wrong after a couple of minutes of showing 40 mph, I do not know. It did this repeatedly. The rental car did not have GPS in it. So, it could not have been that it was crosschecking against that info.

    May be it said – “Well I give up. I am saying stay below 40 and you are driving over 70. What is the point of even me saying something?”


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