# Weight activated stoplights and not enough weight **URL:** \ **Category:** Factual Questions\ **Created:** [April 12, 2005, 10:18pm UTC](https://boards.straightdope.com/t/weight-activated-stoplights-and-not-enough-weight/299029 "2005-04-12T22:18:04Z")\ **Posts on this page:** 8\ **Page:** 2
**Author:** ![aerodave](https://avatars.discourse-cdn.com/v4/letter/a/f08c70/32.png) [@aerodave](https://boards.straightdope.com/u/aerodave)\ **Post date:** [April 13, 2005, 2:18pm UTC](https://boards.straightdope.com/t/weight-activated-stoplights-and-not-enough-weight/299029/21 "2005-04-13T14:18:36Z")
> [@Scarface Z](#): > > BTW, the scientific term for this is magnetic flux. Magnetic flux is a change in a magnetic field, and changes in a magnetic field induces a current through a circuit. I hate to nitpick (no I don’t 😛 )… Magnetic flux is the product of magnetic field density and a reference area. The area of interest may be the area enclosed by the loop under the pavement. Electric current is induced by changes in magnetic _flux_ over time. The field strength can stay the same, and you can still have a change in flux if the area changes. An expanding conductor loop in a constant field is a good example of this. I’m not saying that’s how the vehicle sensors work…but it’s an interesting point. ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [April 13, 2005, 2:36pm UTC](https://boards.straightdope.com/t/weight-activated-stoplights-and-not-enough-weight/299029/22 "2005-04-13T14:36:08Z")
Your rights are the same at any intersection that has equipment that is not functioning properly. Look both ways, wait for it to clear and proceed. Notify local agency of the problem. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [April 13, 2005, 6:45pm UTC](https://boards.straightdope.com/t/weight-activated-stoplights-and-not-enough-weight/299029/23 "2005-04-13T18:45:32Z")
> [@Dag Otto](#): > > The outline of the sensors at these stoplights can be seen in the pavement, usually as a what looks like a rectangular shape cut in the asphalt. The trick to setting these sensors off with a bicycle is to position the bike over one side of the cutout. Roll your wheels right over the line in the pavement. I just wanted to emphasize this. This is the only way I can get some of the lights around here to work. ---
**Author:** ![Bookkeeper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bookkeeper/32/281_2.png) [@Bookkeeper](https://boards.straightdope.com/u/Bookkeeper)\ **Post date:** [April 13, 2005, 8:16pm UTC](https://boards.straightdope.com/t/weight-activated-stoplights-and-not-enough-weight/299029/24 "2005-04-13T20:16:10Z")
The local sensor-activated lights have a row of three yellow disks to show motorcyclists and bicyclists the best place to stop to ensure the sensor is activated. You might want to suggest this to your local traffic department. ---
**Author:** ![Edward\_The\_Head](https://avatars.discourse-cdn.com/v4/letter/e/ce73a5/32.png) [@Edward\_The\_Head](https://boards.straightdope.com/u/Edward_The_Head)\ **Post date:** [April 13, 2005, 8:55pm UTC](https://boards.straightdope.com/t/weight-activated-stoplights-and-not-enough-weight/299029/25 "2005-04-13T20:55:23Z")
I’ve always found that getting close to the cuts in the ground work best, if it works at all. Though the problem there is that you’re either getting close to or actually sitting/putting your foot down in the oil. Besides the Tennessee law, there are a couple of other states in the last couple of years that allow motorcyclists to go through the red light if it does not activate for you. I’m sure that if you proceded with care you could get away with it in most any state unless they really wanted to give you a hard time. ---
**Author:** ![Scarface\_Z](https://avatars.discourse-cdn.com/v4/letter/s/50afbb/32.png) [@Scarface\_Z](https://boards.straightdope.com/u/Scarface_Z)\ **Post date:** [April 13, 2005, 11:10pm UTC](https://boards.straightdope.com/t/weight-activated-stoplights-and-not-enough-weight/299029/26 "2005-04-13T23:10:53Z")
> [@Chronos](#): > > And the detectors don’t detect magnetic materials per se (although a large magnet probably would show some sort of signature), but conductive materials. Since aluminum is a better conductor than iron, it should, if anything, be even more easily detected. Both theory and annecdotal evidence indicates that laying a bicycle flat on its side will make it more detectable; I’ve never gotten this to work, but that may just be because I was in the wrong place (the intersection I tried it at was one of those new construction ones without grooves). Really? I was told by my physics prof. way back when that the way they detect the metal is that the metal becomes magnetized by the magnetic field of the circuit, and that the magnetic field of the car changes the magnetic flux, which therefore changes the current in the circuit, rendering it detectable. The links seemed to support this: > [@](#): > > The magnetic field induces an electrical voltage in the wire that is counter to the voltage of the circuit as a whole. This significantly alters the flow of current through the circuit. > [@](#): > > The huge mass of metal that makes up your car alters the magnetic field around the loop, changing its inductance. Since iron is so readily magnetized, and aluminum is not, I thought there might be a significant difference for a vehicle with large amounts of aluminum vs. steel. > [@aerodave](#): > > Magnetic flux is the product of magnetic field density and a reference area. The area of interest may be the area enclosed by the loop under the pavement. > > Electric current is induced by changes in magnetic flux over time. The field strength can stay the same, and you can still have a change in flux if the area changes. An expanding conductor loop in a constant field is a good example of this. I’m not saying that’s how the vehicle sensors work…but it’s an interesting point. You’re right, it’s been a while since I had to think about this stuff. Magnetic flux is not the change itself, but changes in flux will induce a current. At least I remembered the word. :smack: ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [April 13, 2005, 11:39pm UTC](https://boards.straightdope.com/t/weight-activated-stoplights-and-not-enough-weight/299029/27 "2005-04-13T23:39:28Z")
> [@Scarface Z](#): > > Really?Since iron is so readily magnetized, and aluminum is not, I thought there might be a significant difference for a vehicle with large amounts of aluminum vs. steel. There might be a difference. The sites I read indicate that they think the thing works by putting a magnetic material in proximity to the street coil thus forming a magnetic core that changes the inductance. If this is true then a car with lots of aluminum wouldn’t be as effective as one of steel and iron. I wonder about this since the magnetic field from the current-carrying coil has to either surround the core or pass through a hole in the core (like the window in a transformer core) to make the magnetic material be a core. It seems to me that the metallic object forms a shorted turn in the secondary of a transformer. This changes the current in the primary [the coil in the street] and because the primary coil voltage stays the same this is an effective change in the primary impedance which would unbalance the Wheatstone bridge. If this is the mechanism then any conductive material will do the job if there is enough of it. ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [April 14, 2005, 4:28am UTC](https://boards.straightdope.com/t/weight-activated-stoplights-and-not-enough-weight/299029/28 "2005-04-14T04:28:52Z")
When I had a motorcycle, I got decent results from stopping on the cut line and rocking the bike from side to side. I often got the light (very briefly) with no help from a car. [Previous page](https://boards.straightdope.com/t/weight-activated-stoplights-and-not-enough-weight/299029.md?page=1)