If you live in a major city, traffic is a common frustration. Idling vehicles harm air quality. Traffic management in dense urban areas is complex with conflicting goals. At intersections, vehicles, bikes, and pedestrians must cross paths safely. Over the years, we've developed ways to manage right-of-way, from signs to roundabouts, but traffic signals are most common.

Cities are like human anatomy: highways are the aorta, small collector roads are capillaries, and arterial roads are medium-capacity connections. Arterial roads use at-grade intersections with interrupted traffic flow. These intersections limit roadway throughput; adding lanes or raising speed limits won't help. Increasing intersection efficiency is key. Most accidents occur here, so engineers analyze designs for safety and efficiency.

Assigning right-of-way is challenging, requiring compromise on space, cost, speed, cycle time, sight distance, traffic types and volumes, and human factors. Intersections must be standardized so drivers know their role. The ideal intersection would cause no interruption, but high-five interchanges are impractical everywhere. Simple signs are cost-effective but can't handle high volume. Traffic signals offer a balance: low cost, minimal space, and handle large volumes with some interruption.

In simplest form, traffic signals have three lights per lane: green for right-of-way, red to stop, amber warns of change. Beyond this, signals can be complex for various situations.