Reliable delivery of emergency messages is critical for VANET safety applications. However shadowing in VANETs can cause these packets to be lost. In this paper we propose a reliable opportunistic broadcast protocol (R-OB-VAN) that overcomes this problem and maintains a high delivery ratio and low average delay. The second contribution of this work is to define a simple but realistic model of shadowing caused by large vehicles and study its effect on the surrounding vehicles traveling on a highway. The results of our simulations show that R-OBVAN delivers the emergency messages both quickly and with a high degree of reliability