  Cooperative Intelligent Transport Systems (C-ITS) make road traffic safer and
more efficient, but require the mobile networks to handle time-critical
applications. While some applications may need new dedicated communications
technologies such as IEEE 802.11p or 5G, other applications can use current
cellular networks. This study evaluates the performance that connected vehicles
can expect from existing networks, and estimates the potential gain of
multi-access by simultaneously transmitting over several operators. We upload
time-critical warning messages from buses in Sweden, and characterise
transaction times and network availability. We conduct the experiments with
different protocols: UDP, TCP, and HTTPS. Our results show that when using UDP,
the median transaction time for sending a typical warning message is 135 ms. We
also show that multi-access can bring this value down to 73 ms. For
time-critical applications requiring transaction times under 200 ms,
multi-access can increase availability of the network from to 57.4% to 92.0%.
