were you charged under S. 149 of the HTA? If i'm reading that section correctly, the only restriction is visibility.
http://www.e-laws.gov.on.ca/html/statut ... .htm#BK225
Driving to left of centre prohibited under certain conditions
149. (1) No vehicle shall be driven or operated to the left of the centre of a roadway designed for one or more lines of traffic in each direction,
(a) when approaching the crest of a grade or upon a curve in the roadway or within 30 metres of a bridge, viaduct or tunnel where the driverÃƒÂ¢Ã‚â‚¬Ã‚â„¢s view is obstructed within that distance so as to create a potential hazard in the event another vehicle might approach from the opposite direction; or
(b) when approaching within 30 metres of a level railway crossing. R.S.O. 1990, c. H.8, s. 149 (1).
(2) Subsection (1) does not apply,
(a) on a highway divided into clearly marked lanes where there are more such lanes for traffic in one direction than in the other direction;
(b) to a road service vehicle where precautions are taken to eliminate the hazard; or
(c) on a highway while it is designated for the use of one-way traffic. R.S.O. 1990, c. H.8, s. 149 (2).
I wouldn't be too worried about upgraded charges because you chose to fight it; if it was S. 149 then the crown will most likely withdraw the charge based on both parties accounts (you + witness)
i think careless would've been the more appropriate charge based on the complaint, and one the prosecution could potentially get a conviction. but S. 149 is very specific about when it does not want you to be left of centre; it does not appear to apply to your case, there are situations with S. 149 where you can be left of centre with approaching vehicles as-long as you're not near a grade, curve or <30m from bridges, viaduct, tunnel, railway crossing.
S. 149 sounds like the plea-bargain charge the crown would reduce to from S. 130 careless driving at trial.