Vince points out the cops responding to a street racing call in such force, when they were supposedly busy with a homicide call, was too fast and orchestrated. Brian probably told them where the street race was and again once it ended, to give him an in by rescuing Dom. A few points clue us in:
A random police car singles out Dom and chases him. Brian then just happens to know where to "rescue" Dom (the location probably being radioed to him via the cop). Brian also would've had to pass the police car to get into the alley, so the police car probably let Brian in.
The police don't pursue Brian after he evades their roadblock, despite easily being able to.
There's also no police presence at his home afterwards, probably so Brian could scope out the place safely.
Just before the first race, Dominic uses a remote control to turn on the stereo in his trunk. Why doesn't he just use the stereo controls right on the center console? Because as we see during the race, that's actually the controls for the nitrous system in disguise.
