Efficient and safe transportation of people and goods are key requirements in a modern economy. Traffic control systems are installed at complex intersections to ensure the safe and efficient flow of traffic. However, there are concerns that an adversary could launch cyber attacks that exploit flaws in traffic control systems to cause mayhem and accidents.This chapter presents a co-simulation framework for cyber-physical systems that enables researchers to execute cyber attacks on traffic control systems and measure their impacts on road traffic. The approach integrates an emulated supervisory control and data acquisition master station with a microscopic traffic simulation tool that provides all the functions of a traffic signal control system. The impacts of cyber attacks on road traffic are measured from the outputs provided by the traffic simulation. Experimental results for a corridor of six coordinated signalized intersections are presented, where the impacts are measured in terms of vehicle travel time and queue length. The results reveal that the physical impacts of compromising a single intersection could be felt at other intersections in the road network. This type of emergent result could only have been observed using a co-simulation framework.