Travelers depend on the U.S. transportation system every day to get to work and to special events, acquire and deliver goods and services, and respond to emergencies. If the system operates as expected and enables travel within a predictable amount of time, we can rely on it. When it doesn#39;t, we can#39;t. This lack of reliability affects our health, jobs, families, relationships, cost of consumer goods, emergency response times, and the amount of time we have for other things. So, does travel time reliability matter? Yes. But what is travel time reliability really, what are the impacts of unreliable travel time, how do we measure it, and what can be done to improve reliability? These are the key questions addressed in this primer.