Luckily for you, this describes most of Bangkok. It is a pretty decentralized city, with pockets of low income house scattered around even the nicer areas of the city (with the exception of say, Ploenchit/Chidlom/Siam).

One area with a ton of pedestrian and car traffic is the Asoke/Sukhumvit intersection. Horrible traffic and floods of people because it a skytrain/underground connection.

Another bad area with a mix of people and cars would be around Phahon Yothin, around Central Lad Prao and Chatuchak Park. There is a weird "5 way" intersection that gets horribly congested during rush hour. Chatuchak is also an interchange between the underground and skytrain.

Finally Siam has a lot of pedestrians and traffic, but they do have pretty decent skywalks for people to coach outlet store walk a fair distance, all the way from National Stadium BTS to Central World and Pratunam.