You unlock the Go Kart Street Races after you complete the 5 Go Kart races at the Carnival.
There are only 3 in total but you have to have unlocked Blue Skies in Chapter 5 to be able to do the 3rd race, best to save this trophy until you have beaten the main game campaign.
The benefit to these races over the Carnival ones is you actually get money for winning these as opposed to Carnival tickets.