Effectively broadcasting emergency messages in vehicular ad hoc networks (VANETs) is an important issue in VANETs. This paper proposes a Dynamic Partition Assisted Broadcast (DPAB) protocol to effectively broadcast emergency messages in VANETs. Adjusting the power level of control messages to estimate the network density and dynamically partition a transmission range into segments are the major techniques in DPAB. To do so can not only find the best vehicle in the furthest possible segment to relay the emergency messages in the network, but also can reduce the interference among vehicles to save the network resource and increase the packet delivery ratio. Based on the mathematical analyses and network simulations, the proposed protocol, DPAB, has a significant performance against the related work in one hop delay and packet delivery ratio.