When approaching a speed limit change along the highway as indicated by appropriate signage, where is the actual speed zone boundary? Am I expected to change my speed once the sign becomes visible to me so that I have reached the new limit by the time I reach the sign, or do I maintain my speed and then increase or reduce it only after I have reached the sign?
Speed Zone Boundary
Imagine a line painted across the highway perpendicular to the edge of the road at the sign. Before you reach that line, you must follow the limit in the zone you are in. Once you have passed the line, you must travel at (or below) the speed of the zone that you have entered.
Although many drivers think that as soon as they are able to see and read the sign it is an indication that they are allowed to speed up, they are not correct. You must pass the sign before speeding up.
The reverse is true though, when you see the sign calling for a reduction in speed, it is wise to begin slowing so that you are travelling at the correct speed before you pass the sign. In fact, there are signs posted to remind you of this.
In the Real World
Drivers typically speed up as soon as they can see the sign and begin to slow after they pass it. This behaviour often creates problems for other road users.
One example is when a reduced speed zone is necessary to allow traffic from side roads to join the major traffic flow. Failing to drive at the reduced speed puts both the driver entering the highway and the driver that failed to slow for them at increased risk for a collision.
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Within a municipality, you can usually rely on looking for the end of the zone as your guide.
But for years, at Boundary & Marine, the Vancouver cops loved hanging out facing west on Marine Drive - three lanes wide, nobody paying attention to the exact legal speed, and no municipal sign indicating the flow speed was not the legal speed, as at that time Vancouver just relied on the MVA rules, so there wasn't any need for additional signage.
It was like shooting fish in a barrel.
Drivers are expected to know the limit in a city is 50 unless otherwise posted, aren't they?