None of those things will work in isolation. To reduce downtown traffic requires a comprehensive plan with multiple modes working together:
1) Much of downtown's traffic is caused by people circling looking for traffic, waiting to park, pulling in and out of parking spaces and blocking traffic by double-parking. If we significantly reduce on-street parking and increase off-street parking (i.e. in multi-level lots), then we would reduce a significant amount of that traffic.
2) In conjunction with this, a concerted effort to reduce "blocking the box" behaviour, like double-parking, parking in no-standing zones, or blocking intersections, etc. would greatly improve the flow of traffic. It would help free up bus lanes and cycling lanes, too.
3) Better snow removal in the wintertime. Many boroughs have cut back in recent years to save money. This causes a ton of traffic, as people can't park, drive, get through snowbound streets, and block roads to dig and shovel. It also prevents buses from getting through, and the lack of reliability of public transit forces more people into their cars.
4) Reduce the complexity of parking signage and laws. I know it's in the city's best interest right now to make it as complicated as possible, since it brings in millions in revenue from parking tickets. But if people have an easier time understanding the parking laws, they won't circle around so much and they won't be forced to move their cars as often, which would cut back on traffic at any given moment.
5) Construction!!! This is maybe the biggest one of all. Our construction zones are poorly planned, our projects drag on and on past the planned completion dates, and detours and re-routings are ill-conceived to the point of absurdity. If we could cut back on the needless amount of road closures, lane closures and detours due to construction messes, traffic would flow much better and we'd have lots less traffic.
6) Fix the metro!!! We've been hearing about signal and control centre and software issues for nearly a year. Enough with the excuses already. People aren't taking the metro if they have a choice, simply because they can't rely on the metro to get them from A to B without breaking down a dozen times a month. If the STM is serious about enticing more people from their cars to the transit system, it has to be reliable, bar none. No private company in the world would get away with the sheer level of incompetence we've accepted from the STM. This needs to be fixed, yesterday.