If you don’t have public transportation that I feel safe having a 10-year-old ride on unaccompanied by an adult then you do not have public transportation. You have a very expensive drug addict shelter on wheels/rails.
I live in Montpellier France and all transit has become free 2 years ago. I didn't notice any change in people's behavior at all. Transit is also free in Talinn IIRC and I wonder if they saw something there?
Then you'll ask for receipts of what they pay to use the roads and they'll be out.
Regarding free transit, I think part of the reason it can fail is the socioeconomic situation. In places that rewards the rich and abandon the less fortunate, it's deemed to fail.
But also, I think that when people have to pay to use transit, they'll be less willing to act as a warden, so if they see someone doing something negative, they'll keep out, thinking that it's the responsibility of whoever is taking their money. Meanwhile, when we see public transit as a common good, we are more willing to act to keep it great.
The area clinics were participating in this, because they could procure "Reduced Fare" passes that they would distribute to mentally ill patients, and we could get unlimited transit rides all month long, as long as we were still checking in, on the regular, with our case managers. Our providers would ensure that they qualified us for the "Reduced Fare" program.
After a while, the bus stops became camping grounds for street people who didn't ride. And the trains became camping grounds for people who needed to sleep. Literally, early in the morning, I would board the train and see people zonked out, with pillows, blankets, the whole bit.
Then a campaign began to clean up behavior aboard the transit system. Riders would need a destination, and fares were checked, and people were booted if they hadn't paid fares. So the vehicles themselves became quite sparse, and safer, and smelled better. But oh, the bus stops again. Everyone camped with impunity at the bus stops, and for paying passengers, it was intimidating just to beg for one place to sit down.
The transit system is undeniably safe. I am sure that 10-year-olds can ride unaccompanied. Any violence or fights, those seem to be between gangstas or people who know one another already, not just random outbursts.
Thankfully, too, they open up centers where people can chill, and get drinks of water and use the bathroom, which is honestly preferable to riding buses on false pretenses.
I also think this is relatively orthogonal to cost; cost is nice because it helps keep the people you don't want in the train (crazy people with knives, people smoking weed in the train, people who yell at everyone on the train and make them feel unsafe, etc.), but it's also important to create at least a small barrier so that people don't 'waste' the transit system! In general, when things are completely free, they will be taken advantage of; even a very small tax / friction helps stop this. If the subway is completely free, there's no reason to not just sit on it all day (taking up space and making it worse for everyone). I think subsidizing the subway is net beneficial given that we subsidize cars and things already with road upkeep and such, but free is not what you want.
What an escalation. If I don’t feel safe having a 10 year old drive a car is that just a drug addict shelter on wheels?
The 17 year old dodging fares becomes the 34 year old doing the same.
The percentage of fare dodgers who aren't pieces of shit in other parts of their life is zero.
Free transit is a downward cycle, where increased ridership just increases costs, creating a negative flywheel.
The positive "flywheel effect" where transport connectivity enables network effects does exist, but it does not follow that free public transport leads to negative effects.