Really solid so far!

Planning a Crazy Taxi style game?

Really nice!

I'd reduce the collision radius by 2 pixels. Feels like I'm driving into invisible walls that surround the visible walls.

Also, since you're using the vertical axis for the buildings, may I suggest something for the car to make it look like it uses the same 2.5D projection? Have four sprites for the car that you draw overlapped:

• first sprite is the wheel level, basically a cross-section of the car just above the axles
• second sprite is just below the windows, showing the hood/trunk (or bonnet/boot if you're overseas)
• third sprite is windows, but no roof
• fourth sprite is the roof

If you stack these up at one-pixel vertical increments, it ought to give your car a vertical component to match the buildings.

(This might not be the look you're going for, though.)

I threw together a quick mspaint example of what I mean:

I'm sure the real thing would require a lot more tweaking.