So there are a few things to consider:
(1) In Ontario, the case itself that the prosecution likes to use (see below: R v Bland) is quite old (1975).
(2) If the officer says he was going the speed limit and you passed him, then this in itself would set the stage that you were speeding, but we do not know by how much.
(3) So then the officer says he "followed for 4 to 5 seconds" so this would set the stage for how much but only IF the officer testifies that he followed you at a specific fixed distance for that 4 to 5 seconds. If officer says he took those 4 to 5 seconds to catch up to you, then that is good for you. The officer needs to pace you at a steady/fixed distance for a period of time to get your speed.
(4) So the only thing left that you can actually challenge is the accuracy of the speedometer. When was the speedometer last checked and calibrated? Probably never. How does the officer know the speedometer is correct? They don't!
(5) The other helpful case could be R v Mahoney-Bruer, 2015 ONSC 1224 (see below) which might help bring doubt to accuracy of speedometer.
Unfortunately, the courts in Ontario have taken the position that if the officer believes the speedometer was correct then it must be so. Of course I disagree with that assumption, but that is what we are dealing with at this time.
Now there favorite caselaw they like to use with regards to speedometers is the Ontario Court of Appeal case R v Bland (1975), 6 O.R. (2d) 54 (not available on Canlii)
It says this:
"Where evidence is given that over a measured level distance the police officer's speedometer recorded steadily at the speed alleged, this is prima facie evidence that the accused was driving at that speed, in the absence of some evidence, elicited either on cross-examination or by defence witnesses, which would suggest that the police speedometer was inaccurate."
"Lord Goddard held that if evidence is given that a mechanical device such as a watch or speedometer (he could see no difference in principle between them) recorded a particular time or a particular speed, which it is the purpose of that instrument to record, that can by itself be prima facie evidence, on which a Court can act, of that time or speed. He said that where a very small period of time, such as one-half a minute, was involved, the Court might well require evidence as to the accuracy of the clock in question. He also pointed out -- and this is highly relevant here, where the speedometer read 95 m.p.h. but the appellant was only charged with driving at 90 m.p.h. -- that the speedometer must have been very inaccurate if the offence was not committed."
PROBLEMS WITH R v Bland (1975), 6 O.R. (2d) 54:
(1) The quote from Lord Goddard is from a case from something like 1952.
(2) Personally I would disagree that any mechanical device can be assumed to be accurate it is doing what it was designed to do! Any mechanical device can wear out and become inaccurate over time, even if it was accurate to start. This is why older fully mechanical gas pumps still had to be checked for accuracy every couple years.
(3) Since the mid to late 1990's speedometers are NOT mechanical. They are electronic, even the ones that still have needles.
ANOTHER CASE TO CONSIDER
R. v. Mahoney-Bruer 2015 ONSC 1224 (also called R. v. Ontario (Provincial Police)  O.J. No. 972)
Canlii link: http://canlii.ca/t/gght2
At paragraphs  and  it says this:
"Sergeant Martin is a master instructor on the use of radar and laser.
Martin testified that pacing involves following the subject vehicle while the officer maintains a constant distance behind the subject vehicle while noting that speed off of the police cruiser's speedometer. He testified that the speedometer must be checked for accuracy with a radar or laser device."
So IF during cross-examination, the police officer does not mention at anytime that they checked the speedometer for accuracy with radar or laser, THEN during your closing statements you can bring up this case and state that there was no proof of the speedometer being accurate and therefore there should be reasonable doubt as to it's reading.
CONSIDER THESE THINGS...
- Can you convince the Judge or Justice of the Peace that the speedometer is not accurate? Maybe, but most likely they will side with the police and prosecutor.
- Can you get the officer to admit that he did not pace you at fixed distance for a specific period of time? Maybe, but again depends on officers notes and cross examination technique.
- Did the officer fail to give any testimony that speedometer was checked for accuary against radar or laser/lidar? If so, then will the JP accept the R. v. Mahoney-Bruer case as reasonable doubt to speedometers accuracy?
- If ticket was reduced, and you lose at trial (in Ontario), your ticket will be raised up to the higher speed that the officer testifies about.
+++ This is not legal advice, only my opinion +++
Your post is quite timely, though. When I was pulled over for speeding last November, the officer was in front of me the whole way. Unless he used rear-racing radar (I won't know until I receive my disclosure), I guess he can't use the pacing argument and he'll have to find some other way of proving I was speeding.
Anyways, an officer can pace you from in front as long as they testify that they maintained a fixed distance for a period of time.
As far as radar goes, the units can have both a front facing and rear facing antenna, they could have gotten you on radar.
A little known fact though, is that moving mode radar going in the same direction, can NOT give a reading of a vehicle going the same speed as the patrol vehicle. The patrol vehicle needs to be going at least 3 kmh faster or slower than the target vehicle to get a reading. Of course you will need an expert witness qualified technician (coming soon) to get on the stand to get them to believe you!
+++ This is not legal advice, only my opinion +++
Thanks for sharing jsherk, good to know!
If the officer tries to talk about pacing, maintaining constant visual contact, etc., I'll go after him on that. Did he look only in his rearview mirror and never ahead? It was morning rush hour; I was hardly the only person driving a car of my description. Between glances in the rearview, cars could easily have changed positions. It was still early enough after dawn for headlights to be required; in that lighting, driving mostly away from the rising sun, all cars look black and all windshields are essentially opaque. How, exactly, did he know the same driver was following him all the time? That may be a reach, but it could work.
Again, I'm only speculating since I haven't received disclosure yet.