The selection of WIM technologies depends on the purpose and usability of the output data, data accuracy requirements, road environment such as pavement type, number of lanes, traffic volume, power and communication requirements, costs of equipment, installation, maintenance and data storage, on-going support from vendors and in-house road agency expertise.
Some broad guidelines for selecting WIM systems include:
- Culway is an Australian system with a relatively low equipment and maintenance cost. Substantial expertise has also been established amongst road agencies over many years. Main Roads WA has sponsored further development and improvements in recent years. Culway is recommended for two lane rural highways and roads in areas where culverts can be installed.
- Bending plate, piezoelectric cable and capacitance pads are currently successfully used in Australia and New Zealand and are suitable for urban and rural double and multilane roads.
- For higher accuracy (but potentially at a higher cost), the bending plate or capacitance pad should be used for urban roads, e.g. urban motorways.
- For temporary or portable applications, an above-ground installation the capacitance pad is recommended.
The table below compares the strengths and weaknesses of WIM technologies.
Table: Strengths and Weaknesses of Five Types of High Speed and Low Speed WIM Technologies
|Technology||Capital cost||Accuracy||Maintenance cost||Strengths||Limitations|
|Bending plate||Moderate||Type I||High
|Frame separates sensors from pavement structure; entire tyre fits onto sensor allowing more accurate measurement of wheel load; extensive industry experience with the technology in US.||Not suitable for spray seal pavements; some systems experience premature failure; lower design life (10 years) than strain gauge (Culway).|
|Capacitance pad||Moderate||Type I||High
|Frame separates sensors from pavement structure; portable operation possible.||Not suitable for spray seal pavements; portable operation possible but subject to errors caused by impact loads and changes in pavement strength.|
|Hydraulic load cell||High||Type III||Medium||Entire tyre fits onto sensor.||High equipment and installation cost; low speed (5 km/h); precise level concrete pavement required.|
|Piezoelectric cable||Low||Type II||High||Easier faster installation than other WIM systems; wide industry experience with the technology in many countries.||Not suitable for spray seal pavements; meticulous installation required; low cost and ease of installation often result in placement in slightly rutted pavements resulting in loss of accuracy; high maintenance costs i.e. 6 monthly calibration and 4 year design life; readings compensated for temperature.|
|Quartz piezoelectric cable||High||Type I||High||
Easier faster installation than other WIM systems; accurate sensor capturing more comprehensive information about the vehicle:
single and group axle/tyre loads
precise vehicle classification
|Not suitable for spray seal pavements; requires multiple sensors per lane; accuracy affected by pavement strength; asphalt acceptable, concrete preferred; not affected by temperature; lower design life (10 years) than strain gauge (Culway).|
|Strain gauge (Culway)||Moderate||Type I||Low
|Widely used throughout Australia and New Zealand; easy installation on box culvert; longer design life (25 years) and lower cost than other systems; suitable for spray seal and some asphalt pavements.||Requires box culvert for installation; less accurate than hydraulic load cell systems; not suitable for concrete pavements.|
Source: Adapted from a VicRoads email on 11 Oct 2010 with table contents based on the Federal Highway Administration's State’s successful practices weigh-in-motion handbook.
In 2008 Transport Certification Australia sponsored research into OBM monitoring as part of the implementation of IAP. Preliminary evaluation showed that most trial systems achieved an error less than ± 2% for a small set of test vehicles.
The future development of OBM monitoring and its impact on fixed-site WIM systems should be noted.
McCall, B and Vodrazka, WC 1997, State’s successful practices weigh-in-motion handbook, Federal Highway Administration, Washington, DC.
Transport Certification Australia 2009, On-Board Mass Monitoring Test Report (Final), TCA, Melbourne, Vic.