When Does a Speed Limit Come Into Effect
On paper, speed limits sound like a pretty simple idea- legislators and experts agree
upon a given speed at which it’s deemed safe to travel through an area and then let
the public know by clearly signposting it.
But a question we’ve received numerous times over the years is when exactly does a speed
limit legally come into effect?
Here now is the answer.
Any signposted speed limit comes into effect at the exact point the sign resides.
In other words, if you’re driving at 30 MPH through a 30 MPH zone and you’re about
to enter a zone that allows you to drive at 50MPH, it is technically illegal to begin
accelerating beyond 30 MPH until you’ve reached the sign indicating the higher speed
limit.
Contrary to what people may try to tell you, by the letter of the law there is no “buffer”
or grace period near a speed limit sign, which is why you’ll notice that when the speed
decreases drastically, you may see a helpful warning sign telling you this before the actual
speed limit sign so you’re not having to suddenly slam on the brakes to get down to
the posted limit before you reach the sign.
While it is certainly true that a police officer pulling you over in such a “buffer” scenario
could be lenient and decide to just let you off with a warning, that’s entirely down
to their own discretion and it doesn’t change that legally you’ve broken the law.
This is true in the US, Canada and the UK, with all of the documentation we consulted
saying the same basic thing- that a new speed limit zone begins at the exact point a sign
is placed.
What does vary between these countries, however, are the tolerances we allow in cases involving
speeding.
In the UK, we have something colloquially known as “the 10% rule” which states that
you can generally get away with exceeding the signposted speed limit by about 10%.
Again though, this is entirely down to the discretion of the police and it’s noted
that technically “exceeding a speed limit by any amount whatsoever is an absolute offence”.
However, official guidelines state that an officer can choose to take no action, or simply
give a stern warning, if a person only exceeds the speed limit within the bounds of 10% plus
an additional 2 MPH, after which they are by the letter of the law required to take
action regardless of how good a mood the officer is in, the severity of which depends on a
combination of their own discretion and how much the individual was speeding by.
In the US, things are a little more complex and the tolerances vary from state to state
with many leaving it entirely up to the discretion of the officer who pulls you over.
Although most states do have speeds which they consider to be “reckless”, this too
varies quite a bit depending on where you are.
Even in states where official tolerances (on freeway speeds at least) exist, there is still
no universal consensus on exactly how much you can go over the limit, though the Federal
Highway Administration recommends that
Speed measurements at the selected road segments shall be used to determine the top 5 percent
of speeds.
This information will be used to establish the enforcement threshold.
The enforcement threshold should never be less than 5 mph above the new posted speed
limit.
This top-down strategy will not overwhelm the law enforcement officers, the prosecutors,
or the courts.
This strategy promotes public and court acceptance of enforcement by targeting only the most
egregious violators.
The overall goal of the enforcement efforts is motorist compliance, not issuance of citations.
This unofficial buffer in speed is a good thing because, wherever you’re driving,
your car’s speedometer likely isn’t perfectly accurate, even if you’re just driving it
off the lot brand new with properly inflated stock tires.
With many cars, particularly in the EU, your speedometer will actually be indicating you’re
going faster than you are.
This is because by law in the EU (ECE-R39) the speedometer can never show speeds less
than the speed you are really traveling.
To account for tires wearing down, temperature and pressure variation, varying loads in the
vehicle and the like, manufacturers tend to err on the side of indicating you’re going
faster than you are, but, also by law, must not fudge the numbers more than an additional
10% + 4 km/hr.
According to Transport Canada, there are no such official regulations at the federal level
in North America, though they aren’t wholly correct in that the U.S. does have federal
standards concerning commercial vehicles, not to be off by more than 5 mph at a 50 mph
reading on the speedometer.
The manufacturers in this case will calibrate the speedometer to the stock tires with new
tread and properly inflated.
As the tire changes size for various reasons, the speedometer will be slightly off, with
variations generally within the range of plus or minus 10%, though if you choose to buy
replacement tires with a different size than the factory tires, your speedometer may be
off by even more, making the unofficial speed buffer given by police particularly useful.
As for the argument about whether or or not speed limits work in various scenarios and
how the values are set, that’s a can of worms we’ll save for another article.
So we’ll instead end by throwing some interesting counter-intuitive road related factoids at
you.
For starters, one of the most popular speed controlling inventions of all, speed bumps,
kills numerous people every year by slowing emergency vehicles, according to the Chairman
of the London Ambulance Service, Sigurd Reinton.
This is a notion backed up by a study in Boulder, Colorado that indicated that for every life
saved by various traffic slowing measures like speed bumps, an astounding 85 people
die because of this slowing effect on emergency vehicles whether inherently, like in speed
bumps, or by jamming up traffic.
In addition to that, according to research done on the roads of New Zealand, the common
zebra striped crosswalk, without any additional signaling, actually increases the chances
of pedestrians getting hit by a car by 28% over if the person had just jaywalked.
It is thought this is the case because pedestrians crossing in crosswalks are much less careful
than those crossing the road elsewhere, even to the point that many people observed in
the study didn’t bother to look if anyone was coming before entering a crosswalk.
Another study done in the United States on over a thousand marked and unmarked popular
crossing areas also showed that marked locations had a much higher rate of pedestrian accidents
than unmarked so long as there weren’t any other signals included with the crosswalk,
such as a stop sign/light or flashing lights.
They also found that including a raised “safety” median for pedestrians to stand in the middle
of roads made no difference to the safety of the pedestrians regardless of the number
of lanes on
the road.