We consider vehicular ad hoc networks (VANETs) in which time division multiple access (TDMA) is employed on the control channel. In order to avoid hidden terminal problems, a time slot can be reserved by a vehicle only when the time slot is unreserved by its one-hop and two-hop neighbors. However, frequent topology changes in VANETs may lead to conflicts between time slot reservations, causing reselection of time slots by vehicles. This problem is known as merging collision. An analytical model is thus proposed in this paper to evaluate the rate of merging collision incurred by a vehicle. We show that the proposed analytical model can provide very accurate results over a wide variety of vehicle traffic densities, as compared with computer simulations.