If you'd like to try it there is a dedicated free parking lot at the Visitor Centre in Canmore. Get there early as it is usually full by lunch time. From there it's easy to hop on the trail for 20 or so kilometres to Banff uninterrupted by traffic. There is a rest stop with picnic tables and a bathroom at the half way mark. The trail itself is very busy, I passed well over 100 cyclists and a half a dozen runners. They divide the narrow paved trail with a single painted line, so you stick to your side, but it's relatively easy to pass slower riders safely and there aren't really any hills to worry about.
With the popularity of this trail I'm left wishing Vancouver Island where I lived actually carried through on their efforts to create cycling trails up and down the old rail lines connecting communities. I enjoyed cycling without cars so much I'd like to make a trip to other trails like the Kettle Valley trail. These type of safe active outdoor attractions should be a priority for communities as people look to create tourist attractions.
If you do happen to stop in Canmore for the Legacy trail be sure to stop in at the only entirely vegan eatery there, a wonderful smoothie shop I found called Toniq. They have a post workout smoothie that is particularly filling and my favourite a turmeric based smoothie they call "sunshine" if memory serves me. Lastly on a cycling related note, I took highway 22 around Calgary to avoid the city and was stunned to see well over 500 cyclists on the road near Bragg Creek. They were outnumbering cars 5 to 1. I've never seen that concentration of cyclists in my life and since nobody was wearing a bib I don't think it was an event, but it was a very nice weather Saturday. Some day I'll pop out there when I'm in the prairies to see what all the fuss is about, it's beautiful country.