Vehicular Ad-Hoc Network (VANET) is an emerging technology, which provides intelligent communica-tion between mobile vehicles.Integrating VANET with Ubiquitous Sensor Networks (USN) has a great potential to improve road safety and traffic efficiency. Most VANET applications are applied in real time and they are sensitive to delay, especially those related to safety and health. Therefore, checking the ap- plicability of any proposed application is very important. One way to achieve that is by calculating the Round Trip Time (RTT), which is the time taken by a VANET application starting from the initiator node (source vehicle) sending a message until receiving a response from the core network. In this paper, we present a new complete analytical model to calculate the RTT of VANET applications. Moreover, we in- troduce a novel detailed network architecture for VANET applications using the IP Multimedia Subsystem (IMS) as a service controller in the USN environment. To the best of our knowledge, there is no previous published work that either studied the RTT of VANET applications or developed a complete architecture to implement them by integrating VANETs with USNs and IMS. The RTT is calculated by combining two analytical models. Firstly, we developed an analytical model to calculate the time needed for the commu- nication between two nodes on a road