Vehicle infrastructure encompasses a variety of road types including: interstate, arterials and neighborhood streets. Each has different characteristics with varying capacities for congestion. The design of each type is different for the variety of uses.
Congestion is a major challenge in transportation planning. The problem is peaking, where massive amounts of vehicles travel at the same time of day, during morning and evening rush hour. There may not be enough capacity at that time of day, however, the infrastructure cannot be built to provide just for the highest peaks. Infrastructure is already overbuilt in most parts of the country. And, congestion can be a good measure for economic productivity and population growth. Although, congestion should be avoided when excessive.
A road’s capacity is based on the maximum flow, or people passing in a unit of time. Weather impacts speeds as well as drivers and vehicle type. A road’s design largely impacts the flow of traffic. Wider lanes lend faster traveling speeds. Topography also plays a role, in flat broad areas, speeds tend to be higher. On the interstate, 2,200 to 2,400 vehicles are able to pass per lane per hour.
On local arterial roads, streets intersect at grade. Some arterials meet at lights or stop signs whereas others may join in a roundabout. Roundabouts may not necessarily be safer for pedestrians or cyclists, because they keep traffic moving. When there is not enough demand at street junctures, a stop sign is placed. When traffic warrants a signal, signals are installed using a variety of techniques to determine signal length and configuration. Some signals are fixed, meaning the timing does not change. Whereas others are semi-activated, where major and minor streets join. Fully activated signals have sensors on each major intersection to direct traffic appropriately. Some signals are coordinated through signal optimization, which considers the speed of the vehicles and block length in order to keep traffic moving.