My last couple tours, there were some days that I had planned to ride on roads, but when there was a rail trail that was nearby, I chose to ride that instead. Couple of reasons:
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• Traffic, as you cited.
• Rail trails are often flatter than roads. The railroad locomotives could not climb more than very shallow grades. Busy roads with the wider shoulders often have some of the hills excavated and valleys built up to lessen the grades, but still can be quite steep. The low budget rural roads with minimal traffic were built on the cheap and often have the same slopes as the ground, often double digit in percent grade. Tow paths are even flatter than rail trails.
• Often more direct routing on a rail trail.
• About six years ago I had a long distance to ride that day, a bit of fog and drizzle, was on a holiday weekend. I also chose the rail trail that day because I was afraid in the weather conditions that the visibility would be poor on a more busy road with holiday travelers that were not used to that road, thus less safe than on a dry day. In this case, I chose to ride on the gravel and mud trail for perceived better safety. But this is unusual.

That said, I usually assume that when on gravel, my speed is reduced by 10 to 20 percent, so that can be a factor if I am in a hurry.

I recently turned 72, but I might have chosen the gravel trails for the above reasons if I was half my age.

My above comments are for loaded touring with camping gear, in my case with 40 to 57mm wide tires. But if I was credit card touring on 28 or 32mm tires, I might instead prefer the pavement over rail trails.