Did you know even if a sign says "Max Speed 20" you could actually get marked down on your test, or even fail, if you drove at 20mph or less!
Why, because sometimes the signs are just an advisory limit, not a legal limit.
Firstly we must understand the signing system in the highway code, the highway code states that signs that give orders (i.e. Laws) are in circles, a red circle will be something your MUST NOT legally do and a blue circle is something you MUST legally, failure to disobey the signs will result in a fine or points on your licence, as well as the safety issues and a failed test.
Here's how I remember it, firstly lets thing of a colour to tell us of something negative, the most obvious colour is red, as after all red means stop at a traffic light. Alternatively if you are into football we all know who the worst team is, Manchester United (Red), so we can say they are negative! and best team is Chelsea (Blue) so they are positive ;-)
To remember the shape I think of the word "NO", see below:
So anything in a red circle is a big NO, you MUST NOT do it, if it is NOT in a red circle then it is just advice, no laws attached. So to decide if it is suitable to do the advisory limit we need to look a the warning sign and reason attached to the advisory limit and make an educated decision as to whether or not if it is suitable to do the advisory speed.
Here are a few common examples:
1. School Zones
If we look at the sign we can see that the "20" is NOT in a red circle this means it is just advice, NOT a law, so the sign is saying "We advise you to do a maximum of 20mph when the lights are showing, this is due to the risk of children form the school running across the road"
What we need to understand is that the lights on these signs will flash amber during certain times of the day, these times are pre-programmed to be every weekday, typically around 8am - 10am and 2pm to 4pm, i.e. school drop off and pick up times.
Firstly at lets say 2pm the lights may be flashing but there are no children around as they have not finished school yet, so we need to choose to ignore the advice and travel at the normal speed limit, 30mph, if there are no other hazards present. Or it may be 9am on a bank holiday, the lights will flash as they don't regonise a bank holiday, but if there are no children around then we need to choose to travel at the normal speed limit as we were advised to do 20mph because of school children in the area but there are no children as it is a bank holiday. If the area is very busy with school children then it is highly advised you do no more than 20, so not to risk hitting a child if the run out in front of your car.
2. Newly Surfaced Roads
After the council re-surface a road there will initially be loose chippings / gravel on the road surface, which will mean less grip/traction to the road, and there is a high chance you will flick up the loose chippings cause scratches to your paintwork or to your windscreen.
So the council will put up temporary signs to ADVISE you to a max speed of 20mph, if you are driving along the road and you can see and hear there are still loose chippings on the road surface then you should follow the advice so not to damage your, or other drivers, car.
However, it often only takes a few days for the loose chippings to be compacted into the road, and often the council will not have the resources to remove the signs immediately, or they may forget!, so if you are driving along and you hear and see no loose chippings then you must travel at your normal speeds, so not to hold up traffic behind unnecessarily.
3. Speed on Bends
These signs are normally very good advice and it is very rare you would choose to ignore it, as they advise you to do a certain speed on a bend, as in the past people have driven too fast round the bend and had a serious accident.
However, we still must use are own judgment skills, sometimes, rarely, the signs may be wrong and it may advise you to do 30mph but the bend is tighter and you need to do 20mph. It is also important to remember the advise is just the bend that it mentions, so once you have completed the bend the normal speed limit resumes.
4. Advisory Speed Limits on Motorways
This is one you will not come across when learning to drive, as they are just on motorways, so it may at first confuse you, as it did for me when I first saw them, as you will come across it and will not have you instructor to ask the rules.
If we look at the sign you will notice the '50' it is NOT in a red circle, so it is ADVISING us to do a maximum of 50mph, it is NOT a legal limit.
What we need to understand is that motorways are highly monitor by cameras and regulated, which is why they are the safest roads by fewest number of accidents (most accidents happening on country/rural roads). As the motorways are highly monitored, if there is an accident up ahead then the control room who monitor the motorways may put an advisory speed limit to warn people they could be slower moving, or stationary, traffic up ahead. But if you look up and ahead as far as your eye can see and you can clearly see the traffic ahead is maintaining high speed and not slowing then you should maintaining your normal speed (typically 70mph). If you can see the traffic is reducing their speed (Red brake lights), then reduce to the advisory speed so you don't have to brake harshly when you reach the slower moving traffic.