# Comparative Accuracy Analysis of Truck Weight Measurement Techniques
## 1 Introduction
Static scales are considered an accurate weighing method and are used as a reference point while testing and calibrating other weighing systems. The portable scale weighing method is reliable, but the process is time-consuming. Each axle weight has to be measured separately, one after another. It requires an operator (typically a qualified police officer) and a driver to move the truck after measuring each axle. The measured truck has to be parked on a flat surface (no slope) and can be an obstruction to the traffic. The portable scale can be moved from place to place because it is easy to setup. Trucks can also be measured at truck weigh stations. Each weigh station consists of built-in static scales that can weigh standing or very slowly moving vehicles. Truck weigh stations are built off the road, typically on major highways. All the trucks must exit the road and go through the scales monitored by the police operator. However, truck weigh stations often do not capture the heaviest traffic. As the location of these stations is known, so the drivers of illegally overloaded vehicles can take alternative routes. In pavement-WIM stations are another measurement system. The WIM sites measure vehicle attributes in motion, and data is recorded continuously. In this case, sensors are built-in the pavement, and they take measurements of axle or wheel weights, axle spacings, etc. The WIM stations have been collecting data for over 20 years resulting in millions of records for each station. The WIM systems can collect traffic volume and a wide spectrum of load parameters; each record collected by the WIM system includes the date, time, occupied lane and movement direction, and the vehicle data (vehicle class, number of axles, axle weights, wheel loads, axle spacings). The weigh-in-motion technologies are available for permanent or temporary traffic data collection classified in the US by ASTM E1318-09 [1] as WIM from Type I to Type IV, depending on the performance requirements of the WIM system. These systems were designed for long-term traffic monitoring. According to ASTM, the calibrations of weighing systems are performed using test trucks. The WIM systems can be classified as low speed (LS-WIM) or high speed (HS-WIM). LS-WIM systems are more accurate than the HS-WIM, but they require measurements with the vehicle speed only up to 15 km/h, which is not feasible on highways. The LS-WIM systems are equipped with strip and bar sensors, scales, and plates, embedded in a pavement structure. The WIM scales can be divided into the following groups: load cell scales, bending plates, capacitive mats, and hydraulic plates [2]. Several types of WIM sensors vary by function (weighing sensors or axle detecting sensors), accuracy, durability, width (plate or strip), price, and requirements for pavement surface. There are three types of piezoelectric WIM sensors: piezoceramic, piezopolymer, and piezoquartz. The first two types are temperature-dependent and mostly used for vehicle count and classification. Piezoquartz WIM sensors are extensively used in frequent freeze-and-thaw cycle regions because of their low sensitivity to temperature variations [3]. The piezoelectric sensor function is based on the difference in voltage due to the applied force. The calibration procedure verifies the force-voltage relationship. However, this type of system is only accurate in dynamic load, while for static or slow-motion speed measurements can produce a substantial error. Load cell-based WIM systems utilize a similar mechanism as bending plates. The weight sensor is usually a strain-gauge type, which converts the applied force into a proportional electrical signal. The load cell WIM systems are commonly used in conjunction with the inductance loops to eliminate incorrect records and activate the principal system [4]. Virtual weigh-in-motion stations are an additional type of WIM. They utilize practically the same WIM scale system along with the digital cameras and software to process the visual information in real-time.