Click for next page ( 18

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 17
17 CHAPTER 2 TYPES OF EQUIPMENT This chapter presents an introductory summary of the types inductive loops in series (dual loops) allows for classification of equipment that are available for collecting classification based on overall vehicle length, but does not allow for classi- counts and for weighing vehicles in motion. For this purpose, fication using the FHWA's 13-category, axle-based scheme. the authors categorize equipment by the type of data collected: Short-duration portable vehicle classification counts; 2.1.1 Short-Duration Classification Counts Continuous (long-duration) vehicle classification counts; Short-duration, weigh-in-motion (WIM) data; and Short-duration counts are the most common of all classi- Continuous (long-duration) WIM data. fication counts. Prior to the mid-1980s, classification counts were almost always collected manually by roadside observers. In addition, the classification technologies are further dif- Visual observation allows a wide variety of classification ferentiated by whether the sensors are placed in or on the road- schemes, including those based on body type and those based way surface (intrusive sensors) or whether they are placed on vehicle configuration and number of axles. However, above or beside the roadway (non-intrusive). Vehicle classifi- because manual observation is expensive, highway agencies cation can be performed using either intrusive or non-intrusive have transitioned to automated data collection. Since the mid- sensors, although the style of sensor used affects the data 1980s, most classification data have been collected using available for classifying vehicles and thus the definition of portable sensors placed on top of the roadway surface. This vehicle categories into which vehicle counts are placed. On the choice of technology means that most classification counts other hand, current WIM technologies all require on-surface or now use axle- or length-based classification schemes. How- in-pavement sensors. ever, further advancements in technology, as well as limita- tions in the more traditional data collection technologies, have encouraged highway agencies and vendors to experi- 2.1 VEHICLE CLASSIFICATION ment with portable versions of non-intrusive sensors. Short-duration classification counts are collected at a wide Vehicles can be classified using any one of several cate- variety of locations. In addition to collecting accurate data, gorization schemes, and alternative schemes often use dif- the technology used for short-duration counts must be easily ferent characteristics to differentiate between vehicles. The moved from location to location, be easy and safe to place, most common classification schemes are based on have portable power supplies that can keep the equipment operating for the periods desired, and be relatively inexpensive. Number and spacing of axles, Short-duration counts are most commonly collected for peri- Total vehicle length, ods of 24 or 48 hours, although some highway agencies attempt Body or trailer type, to collect as many as seven consecutive days of such data. Vehicle weight, or Portable sensors that are commonly used for collecting Engine/fuel type. vehicle classification data include Most technologies can collect some but not all of these dif- Road tubes, ferent characteristics. Thus, if a specific classification scheme Piezoelectric sensors, is required, it is important to select a data collection tech- Fiber-optic cable, nology that can collect the vehicle characteristics that define Portable inductance loops, and that scheme. Similarly, if a specific technology must be used Magnetometers. because of some other constraint (such as environmental factors or pavement condition), it is important to understand The first three of these types of sensor provide information the restrictions that the use of that technology places on the sufficient for use when classifying vehicles into the FHWA's classification scheme. For example, use of two conventional 13-category system, but inductance loops and magnetometers