交通工程专业英语,Chapter 4 Traffic Studies,Chapter 4 Traffic Studies,A well-designed, well-executed traffic study is critical in the decision-making process for many transportation-related projects and it is a basic in traffic engineering. Traffic study is conducted for many reasons and applications, such as managing the physical system, calibrating basic relationships or parameters, assessing potential impacts, evaluating facility or system performance, and so on.,4.1 Objectives of Traffic Studies Traffic engineers collect data for many reasons and applications: Managing the physical system. Physical inventories of physical system elements are always needed. These include traffic control devices, lighting fixtures, and roadways. These inventories help assess which items need to be replaced or repaired, and on what anticipated schedule. Investigating trends over time. Traffic engineers need trend data to help forecast future transportation needs. Parametric studies on traffic volumes, speed, and densities help quantify current demand and assess operating quality of traffic facilities. Accident studies reveal locations with problems that must be addressed and mitigated. Establishing trends allows the traffic engineer to make needed improvements before an obvious deficiency in the system manifests itself.,Understanding the needs and choices of the public and industry. The traffic engineer must have a good measure of how and why people travel for planning and development purposes. Studies of how travelers make mode choices, time of trip decisions and other judgments is critical to understanding the nature of travel demand. Studies of parking and goods delivery characteristics help plan facilities to effectively handle these demands. Calibrating basic relationships or parameters. Fundamental measures, such as perception-reaction time, discharge headways at a signalized intersection, headway and spacing relationships on freeways and other uninterrupted flow facilities, and other key parameters and relationships must be properly quantified and calibrated to existing condition. Such measures are incorporated into a variety of predictive and assessment models on which much of traffic engineering is based.,Assessing the effectiveness of improvements. When improvements of any kind are implemented, follow-up studies are needed to confirm their effectiveness and to allow for adjustments if all objectives are not fully met. Assessing potential impacts. An essential part of traffic engineering is the ability to predict and analyze projected traffic impacts of new developments and to provide traffic input to air pollution models. Evaluating facility or system performance. All traffic facilities and systems must be periodically studied to determine whether they are delivering the intended quantity and quality of access and/or mobility service to the public.,Traffic studies provide the underpinning of all traffic planning, design, and analysis. If the data is not correct, then the traffic engineering that is based on it must be flawed. Some of the tasks involved in data collection, reduction, analysis, and presentation are somewhat mundane. Focused attention must be paid to these tasks, however, if the final outcome of traffic engineering is to be effective.,4.2 Types of Studies It would be literally impossible to list all of the studies in which traffic engineers get involved. Some of the most common include: Volume studies. Traffic counts are the most basic of traffic studies and are the primary measure of demand. Virtually all aspects of traffic engineering require volume as an input, including highway planning and design, decisions on traffic control and operations, detailed signal timing, and others. Speed studies. Speed characteristics are strongly related to safety concerns and are needed to assess the viability of existing speed regulations and/or to set new ones on a rational basis.,Travel-time studies. Travel times along significant sections of roadway or throughout entire systems constitute a major measure of quality of service to motorists and passengers. Many demand-forecasting models also require good and accurate travel-time measure. Delay studies. Delay is a term that has many meanings, in essence, it is the part or parts of travel time that users find particularly annoying, such as stopping at a traffic signal or because of a mid-block obstruction. Density studies. Density is rarely directly measured but is computed from measurements of flow rate and speed at important locations. Modern sensor technology can measure “occupancy” over a detector, which can be converted to a density.,Accident studies. As traffic safety is the primary responsibility of the traffic engineer, the focused study of accident characteristics, in terms of systemwide rates, relationships to causal factors and at specific locations, is a critically important function. Parking studies. These involve inventories of parking supply and a variety of