Try to determine the distance between camera and vehicle, based on the angle Alpha of the vehicle relative to the center of the circular section. There are two angular velocities involved, and the circular angle Alpha dominates in that portion of the path. Find the distance r where, (a < r < a+b), and the height Y of the vehicle on the circle is the sine of the angle Alpha multiplied by b, the horizontal addendum Z to distance a is the cosine of the angle Alpha multiplied by b, and Theta = inverse tangent (Y/a+Z) with Y = b(sine Alpha) and Z = b(cosine Alpha). Theta = inverse tangent {b(sine Alpha) / [a + b(cosine Alpha)]}.
With a constant tangential velocity V, the values of Y and Z are accelerating as the angle Alpha changes. You need to solve for Y and Z as a function of Alpha due to known tangential velocity V, plug those values into the equation for Theta, and take the derivative of Theta.
This is a piecewise problem, since the solution is different for the straight section and the circular section. For the straight section Y is a constant, and Theta is a function solely of distance along the x axis. The angular velocity of the camera is a derivative of Theta, where theta is the inverse tangent of Y divided by X, and X is the integral of the speed V of the vehicle.