Transit systems in the US need some payment technology upgrade.
On a couple of occasions, I haven't been able to board a bus due to not having exact change. Once, I was with kids & family visiting NYC. We were planning to take a public bus from a suburb to the city. The fare turned out to be $16 for all of us, but I had $11 and some $100 bills. Guess what, I couldn't board the bus as they can't return change. The bus driver cannot accept partial fare from me. I couldn't give up 80+ dollars as change. If I had $20, I wouldn't mind not getting back $4.
I wondered why they cannot start accepting credit cards, Apple Pay, Android Pay or any reasonable alternative. I then had to find a train station nearby which could accept credit card.