Careless driving is one of the most severe charges under the HTA and also one of the hardest to prove. The mere fact you were in an accident shouldn't trigger the charge. It should be a marked departure from due care and attention.
Police are infamous for over using this charge. They come across the accident scene, somebody caused the accident, they should be charged with something. Unless the cop is an accident reconstruction expert, there will be little evidence to warrant the charge.
Cuthbertson J did an excellent job describing the essential elements of the charge in R. v. Morgan, 2008 ONCJ 336.
And as always, the best defence is a solid disclosure request. If that does not provide the essential elements to meet the charge, argue improper disclosure (as opposed to going to trial and arguing non-suit).
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Too technical? My bad.
1a. request disclosure (evidence, witness info, witness will say statements, criminal records, etc.) AND
1b. ask for an explanation and clarification of the charge: why was I charged, what evidence do you have that I have committed an offence?
2. When you get the accident report (typed please), it will not give a good indication of why your were charged.
3. Re-request disclosure noting the essential elements of the charge and the reason why you were charged are missing.
4. File an application for a stay under s. 24(1) based on the fact that you were not provided with the evidence against you (a section 7 Charter violation).
5. If it comes to trial, after the Crown has rested, you make a motion of non-suit based on the fact the Crown has not proven the charge.
Finally if you don't win #4 or #5 above, then...
6. Get up on the stand, note that this is a strict liability charge and say basically what is described in paragraph [14] of the case I cited above:
Where the defendant can show, in circumstances that might be reasonably be true, that the defendant was faced with an unexpected, unforeseeable sudden circumstance, and that he reacted as best any reasonable, prudent driver could, this should raise a reasonable doubt.