What is the main topic of the conversation?

M: Hey Teresa! thanks for agreeing to help me review all this history material.
W: No problem, Bob. So do you want to start with the stuff missed yesterday? They are part about urban problems in the colonial period?
M: Yeah, that’ll be great.
W: Ok. Well, when the British were building cities in American colonies, they had to figure out how to make the cities run smoothly, right? Like... take traffic for instance.
M: Traffic? They had traffic that then?
W: Well, not the same kinds of traffic cities have today. Back in 18th centenary there were not any automobiles like there are now. But there were other vehicles like coaches and ears, you know, the kind had been pulled by horses say. Only a few people could afford to have their own coaches then. But there were more and more public coaches. And cars, they were a big problem cause car driver usually overloaded their cars. And these oversize clumsy cars would slow down the traffic, and when you added to that all the farm animals.
M: Farm animals? In the cities?
W: Lots of them, cattle, pigs and other live stuck rolls in the streets. And they really caused trouble, although the pigs at least had good reasons for being their.
M: They did?
W: Yeah, the town’s people used pigs to clean up the garbage that was thrown into the streets. But then the pigs interfered with traffic.
M: So what do the colonists do?
W: Well, they passed traffic laws. Like One that said, if your animals were found wandering in the street, they could be seized. Of course people who owned pigs didn’t want to keep them penned up because the pigs were so good at cleaning up the streets.
M: Sounds like a good idea letting them clean up the garbage. But I cannot see how pigs lying around in the streets could really slow down the traffic.