Interstate highways span the continental United States from coast to coast and from Canada to Mexico, moving people and products throughout the nation. Highway systems honeycomb cities and states. On a highway network, when traffic volume becomes great enough that the traffic stream slows as road use increases, traffic congestion is the result. As congestion builds and traffic demand approaches the capacity of that particular network, traffic engineers begin to plan for ways to relieve the congestion before the demands of traffic density exceed the highway’s capacity and it reaches saturation.
Currently half of all U. S. traffic congestion is of the reoccurring kind, due to sheer volume of traffic. Most of the remaining traffic congestion is the result of incidents like stalled cars, accidents, roadwork and weather-related events. Something as simple as a driver braking hard in an otherwise smooth traffic flow can cause a cascading failure that leads to traffic back up.
Traffic engineering analyzes the behavior of vehicular traffic by studying the interactions between vehicles, their operators, roadways and highway infrastructures (traffic control devices, ramps, signage and such). The goal of their analysis is to understand and develop successful road networks through which traffic moves efficiently and with minimal congestion difficulties.
The goal of a well-constructed highway is good traffic flow. The study of its traffic stream parameters informs civil engineers about the nature of a specific highway’s traffic flow and help detect variations in flow characteristics when they occur. All the data collected informs highway designs, improvements and changes.
Traffic Stream Parameters
The traffic stream is affected by vehicle and driver (human) behaviors. Traffic stream parameters may characterize all the traffic (macroscopic parameters) or just one vehicle within the stream with respect to other vehicles (microscopic parameters).
Macroscopic traffic stream parameters can be classified as:
- Quantity measurements: density and flow/volume
- Quality measurements: speed
Microscopic traffic stream parameters can be classified as:
- Separation measurements: time headway and space headway
The key traffic stream parameters are flow/volume, speed and density, and time and space headway.
Speed is a quality measurement because the drivers care more about making their trip quickly from one point to another than they care about the design of the highway that takes them there. There are five speed types measured by traffic engineers.
- Time mean speed: The time mean speed measures the mean speed of vehicles over a specific period at a specific point on a highway.
- Space mean speed: This measurement is related to the length of a particular highway or section of a highway, or even a lane of that highway. The mean speed of the traffic at a given moment on that section is the space mean speed.
- Running speed: is the average speed a vehicle maintains while it is moving over a particular highway.
- Journey speed: is the amount of distance from point A to point B divided by the total time it took to get there, including any time used for stops. If the two speeds are the same, travel conditions were comfortable.
- Spot speed: is a vehicle’s instantaneous speed at a specific location. Traffic officers use radar speedometers to measure spot speed when issuing speeding tickets. Spot speed is key information for use in designing the geometry roads such as horizontal curves and different elevations, where to place speed zones for driving safety, and the placement and size of signage. Spot speed data is basic information data for accident analysis, road maintenance and solving problems of traffic congestion in the traffic engineering field.
Traffic volume (also known as flow) is the number of vehicles that pass a designated point on a highway, road or lane during a particular time interval. Vehicle volume varies even by seasons of the year. Weekdays show different patterns from weekends. Summer patterns are different from winter. When comparing volume day to day, however, route patterns often show much similarity and allow for the making of predictions. The most significant variation of time and volume of all is that of from hour to hour (i.e., rush hour). All of these are used to determine the designs of highways and their related structures and signage. Hence, volume (or flow) is considered the most important traffic stream parameter.
Density refers to the number of vehicles filling a highway and is the measurement most closely related to traffic demand. The density of vehicles in the traffic stream affects a driver’s ability to maneuver. The traffic stream’s characteristics of time and distance headway are affected by density. Anyone who has maneuvered in bumper-to-bumper traffic understands.
Interventions to Moderate and Correct Traffic Flow
Heavy traffic means more wear and tear on highway infrastructure. Heavy trucks and especially overweight trucks (operating with special permits) weaken the steel and concrete of bridges and highway structures. Even with the improvement in emissions from cars seen in the last decades, air pollution continues to be an effect of heavy traffic. Federal legislation and state laws and enforcement work to support changes on the national and state levels to correct these problems. Weigh stations to check trucks and fines plus road taxes help trucking companies pay their share of infrastructure assistance for their heavy use of highways.
Ramp traffic signaling devices that switch from red to green during heavy traffic times, regulating the spacing of cars (called a “drip feed”) entering limited-access highways, work to prevent the “jockeying for position” that can cause traffic snarls and accidents. Three lane roads with a reversible lane help the flow of traffic in metropolitan areas during rush hour by using the center lane as “inbound” in the morning and “outbound” in the evening. Interstates are adding HOV (high occupancy vehicle) lanes to encourage carpooling and reduce the number of vehicles on them. Park and Ride facilities in key suburban centers provide places for carpoolers to meet and ride share.
HERO® Brigade specially equipped trucks cruise the Interstates in some cities to assist disabled motorists and get stalled cars out of the traffic flow. Metropolitan sections of Interstate highways have designated “Accident Investigation” shoulder parking areas where accident victims who are uninjured and whose cars are not disabled should move their cars out of the flow of traffic to wait for the police to come and fill out their accident reports. Digital highway signs with real-time “decision quality” information provide information about traffic delays and travel times for the stretch of highway ahead. Traffic engineers time traffic signals according to the needs of local traffic patterns.
Police, sheriffs and state highway patrol officers routinely patrol the highway systems in order to identify and diffuse problems with the human element of traffic; the drivers. Speeds are monitored, DUI checkpoints are used, and lawbreakers are pulled over and ticketed or arrested. Cases of road rage are stopped and investigated. Frequently just the heavy presence of law enforcement at various visible spots along the highway systems during heavy volume traffic times (such as holiday weekends) serves as a deterrent to speeding and other driver-related traffic problems.
Designing, building, maintaining and supervising the country’s Interstates and highway systems is the work of civil engineers of the federal government as well as state and local governments. Highway safety is supervised by law enforcement agencies as well as federal and state Departments of Transportation. Correct use of these valuable infrastructures as well as driver courtesy and safety is the responsibility of everyone who drives a vehicle on these roads.
Photo Credit: NYTimes.com