One man was driving a Prius while the other man was driving a Ferrari. From TMZ:

Sources tell us ... Keyshawn -- a former NFL superstar -- had just left a party Monday night inside of his exclusive gated community in Calabasas (with his child in the car), when Bieber zoomed by in his Ferrari at breakneck speeds.

We're told Keyshawn was furious -- feeling Justin could've killed someone -- so he brought his kid home, then chased after the singer in his Prius (yes, a Prius chasing a Ferrari).

When KJ got to Bieber's pad -- he blocked Justin's Ferrari in the driveway and got out of his car to confront Justin face-to-face ... but Justin ran inside of his house and refused to come out.

This begs several questions, the first of which is, what the hell is Keyshawn Johnson doing driving a Prius? 

The second question: how bad is Bieber's driving? So bad that Eric Dickerson is tired of it. Dickerson Tweeted that Bieber needs to "slow his ass down"

Slow you ass down, Bieber. Ya dork.

And Keyshawn, get a new car.