I wouldn't worry about the lanes being too wide, unless one is modeling a specific site. Many US roads didn't have fog lines until the 70s(?), so any excess pavement could be considered the shoulder. And lane widths have always been minimum standards, rather than fixed widths.
The 1961 "Manual of Uniform Traffic Control Devices" calls for the "stop line" to be ten feet from the near rail. I would think that, for higher rail speeds, that would probably be further back. The BN once recommended that, to be safe, one should stand 20 ft from the near rail, plus one foot for each mile per hour of train speed. If there's room, I'd recommend 20 feet for a model mainline crossing.
