Discussion
firstly hello everyone!
next, I recently enquired via the West Midlands Police website regarding the law relating to speeding
I asked them if a vehicles speed was still measured as an average over a measured quarter of a mile, the reply I received was "yes, the law has not changed"
I just wondered how then for example a radar gun can get your average speed over a measured quater of a mile as it is not pointed at your vehicle for anything like the said distance?
many thanks
next, I recently enquired via the West Midlands Police website regarding the law relating to speeding
I asked them if a vehicles speed was still measured as an average over a measured quarter of a mile, the reply I received was "yes, the law has not changed"
I just wondered how then for example a radar gun can get your average speed over a measured quater of a mile as it is not pointed at your vehicle for anything like the said distance?
many thanks
Not that I have any idea on whether this correct or not but I would imagine it's because radar (or laser, whichever is used in the cameras) is not allowed to be used in vehicles (saw that in another thread on here somewhere). So although a camera can get an instantaneous reading of your speed, the traffic cars have to measure your speed in another way, and without being able to take a "snapshot" of your exact speed, the most accurate method would be to take the average between 2 points. Like I said, have bo idea whether or not that's it but it sounds good to me! 
Thanks centurion07 for your reply,hmmmmmmmm maybe then it could be worth some more rooting on the web just in case, (imagine the outcome if after all, speed camera evidence had never been legal!)
The reason I asked the question in the first place was because when I have seen a camera, there is one detector strip in the road then, about 100 yds further on the camera and no way is that the said quarter mile distance between the two points to take the AVERAGE speed as the law states
many thanks once again
The reason I asked the question in the first place was because when I have seen a camera, there is one detector strip in the road then, about 100 yds further on the camera and no way is that the said quarter mile distance between the two points to take the AVERAGE speed as the law states
many thanks once again
safetyfirst said:Surely the difference is in the question. To take the average you need some time (equates to distance). Cameras (laser/radar) record speed at an instant, and the double flash is to provide proof (by distance travelled).
Thanks centurion07 for your reply,hmmmmmmmm maybe then it could be worth some more rooting on the web just in case, (imagine the outcome if after all, speed camera evidence had never been legal!)
The reason I asked the question in the first place was because when I have seen a camera, there is one detector strip in the road then, about 100 yds further on the camera and no way is that the said quarter mile distance between the two points to take the AVERAGE speed as the law states
many thanks once again
I assume that a laser gun takes an average speed, although it would be over an extremely short distance.
s = ut plus 0.5a(t^2)
where:
s = distance travelled
u = initial velocity
a = acceleration
t = time
Assuming acceleration = 0 (accelerate as fast as you like, it's the speed that gets you nicked), therefore 0.5a(t^2) = 0, gives:
s = ut
The gun fires many laser pulses at a predetermined time gap...let's assume 100 pulses and 0.001 seconds apart.
The pulses are reflected back and the distance (s) can be calculated using s = ut (u = velocity of light and t = time taken for pulse to return). This distance is the distance from gun to car and back, just divide by 2 for "gun to car".
You now have 100 distance measurements over 100 x 0.001 seconds duration.
distance0 - distance99 = distance travelled (s)
t = 100 x 0.001
s = ut -> u = s/t -> velocity = distance / time
Out pops your speed, erm, sorry officer, I wasn't going that quick, the speed of light must have fluctuated ;-)
s = ut plus 0.5a(t^2)
where:
s = distance travelled
u = initial velocity
a = acceleration
t = time
Assuming acceleration = 0 (accelerate as fast as you like, it's the speed that gets you nicked), therefore 0.5a(t^2) = 0, gives:
s = ut
The gun fires many laser pulses at a predetermined time gap...let's assume 100 pulses and 0.001 seconds apart.
The pulses are reflected back and the distance (s) can be calculated using s = ut (u = velocity of light and t = time taken for pulse to return). This distance is the distance from gun to car and back, just divide by 2 for "gun to car".
You now have 100 distance measurements over 100 x 0.001 seconds duration.
distance0 - distance99 = distance travelled (s)
t = 100 x 0.001
s = ut -> u = s/t -> velocity = distance / time
Out pops your speed, erm, sorry officer, I wasn't going that quick, the speed of light must have fluctuated ;-)
tja said:
I'm guessing that you'll have to look up the actual wording of the specific law. It might just say "over a measured distance"...which could mean 3mm
But if two readings are taken with laser then with would be over a measured time and not distance. Depending on the speed you could travel between 1mm and 5 mm
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