Mental note

Don't practice physics without a license in Manitoba.  How so?  Apparently, a retired engineer is in trouble for saying something that I've noted as well; short yellow lights--in this case, 4 seconds or less--are a safety hazard.


Now is there perhaps a scientific reason that a retired engineer needs a government license for this?  Really, no; this kind of calculation is something that anyone who remembers first semester physics ought to be able to figure out.  When the light turns yellow, if you're within a certain distance of the light, you need to stop, and it takes about a second for your foot to find the brake.  So when your foot hits the brake, you've got (n-1) seconds to stop, and at 50mph (or 88'/s), that's 264 feet.


Assuming an approximation of constant deceleration, you find that requires about six seconds to decelerate 88'/s, or your deceleration is approximately 15'/s^2, or pretty close to half a g.  If you look up actual stopping distances and times assuming less than optimal conditions, you quickly find that short yellow lights are indeed a safety hazard, especially on wet, snowy, icy, or dusty roads.  Even the best tires only allow deceleration of about a g (force of gravity) in perfect conditions. 


If you wonder why our first amendment is so important, read this again.  There is no reason anyone ought to be punished for pointing out the consequences of Newtonian physics, even if the target is government.