Phong: Mum, I'm going to the park with my friends. Can I borrow your bike?
Mum: Sure, but be careful. You know how busy the streets are these days.
Phong: I know, Mum. There are so many cars and motorbikes. It's hard to cross the road sometimes.
Mum: When I was your age, things were very different. There were not many vehicles on the streets. Most people rode bicycles or walked.
Phong: Really? That sounds nice. Did you have a bike, Mum?
Mum: Yes, I did. It was a gift from my parents on my tenth birthday. I was so happy. I used to ride it to school every day with my friends.
Phong: Did you have to wear a helmet, Mum?
Mum: No, we didn't. Nobody wore helmets back then. It was not compulsory.
Phong: Wow, that's strange. Now we have to wear helmets all the time. It's the law.
Mum: Yes, it is. And it's a good law. It protects you from accidents. You should always follow the traffic rules, Phong.
Phong: Don't worry, Mum. I always do. OK, I'm off now. See you later.
Mum: Bye, Phong. Have fun.

The main idea of the conversation is:

Phong and his mum talk about the differences between the past and the present in terms of traffic and transportation.