  We compare the observed radial velocity of different arm tracers, taken near
the tangent to a spiral arm. A slight difference is predicted by the density
wave theory, given the shock predicted at the entrance to the inner spiral arm.
In many of these spiral arms, the observed velocity offset confirms the
prediction of the density wave theory (with a separation between the maser
velocity and the CO gas peak velocity, of about 20 km/s) - when the observed
offset is bigger than the error estimates. The time to reach the next spiral
arm is given.
