If the witnesses do not appear, and it's a "he said, she said" situation, then the justice will conduct a R. v. W.(D.) analysis, which basically says that if you are credible and you provide your testimony without wavering, the justice must acquit you. If the justice believes you, s/he must acquit. If the justice thinks you are probably or even likely guilty, that is still not enough to convict. The prosecution must prove beyond a reasonable doubt that you committed the offence. This is not an absolute or perfect standard, but it gets very close to being so.
If the police officer testifies, since s/he wasn't there to witness the accident, you can object if s/he says something like "Witness A told me...", because that's considered hearsay.
If you are offered a plea deal for a lesser charge, you may choose to accept it. This will still be a conviction on your record and it may still result in increases in your insurance rates. It may be less likely for a prosecutor to offer a plea deal before trial if all the witnesses have shown up already.
Were you issued an offence notice (ticket) or a summons or an information?