Data provided by VTI vehicle position surveys is only relevant to the right hand side of a vehicles front wheels. No tyre width or type information is available. As parameters such as wheel paths (centre point of a single or double tyre) are considered important, it was decided to test a new system that could also record tyre width and type.
The objective in this project was to develop and test a new position measurement system so that tyre position, tyre width, and tyre assembly type could be determined for every vehicle axle.
The existing measurement technique uses three coaxial cables which are fixed to the road surface in a reverse “Z” type formation. When a tyre runs over a cable, a triboelectric effect produces electron migration in the cable. The charge from the electron migration can be amplified and converted to a voltage pulse. Timings of voltage pulses can be analysed and used to produce a lateral position and code for each passing vehicle. Limitations with the “Z” measurement technique mean that position data is only available for the right hand side of the vehicles tyres. As no information on tyre width or type is collected, exact wheel paths positions cannot be calculated.
To try and overcome these limitations, an additional diagonal cable was introduced in order to capture the left hand side position of each tyre (or pair of tyres). Comparisons, between left and right hand side tyre positions, could then be carried out to produce tyre width values.
The trial survey was carried out on National Road 34, south of Linköping, between 12 and 22 September 2014. The road is a 9 m wide, 1+1 barrier separated type with an AADT of 2675. Approximately 30,650 vehicles were recorded. Around 2900 of these (9.5%) were classified as large goods vehicles (LGVs).
Tyre widths measured during the survey were narrower than expected. For example, an average car tyre should have a surface contact width of around 175 mm. Measured widths averaged around 130 mm.
Forty different types of vehicle were recorded. Eight vehicle types accounted for approximately 89% of all LGV traffic on the E4, Linköping and 83% of traffic on Road 34, Skeda Udde. Vehicle types 234, 323 and 22 were the most common on E4, accounting for 64% of all LGV traffic. Vehicle types 234 and 22 were the most common on Road 34 (63%). Tyre widths measured during the survey were narrower than expected. For example, the average surface contact width of a car tyre should be around 175 mm, however, measured widths averaged around 130 mm. Initial thoughts are that this reduced measurement could be related to the shape of the tyre footprint. If the footprint was rectangular in shape, accurate measurements would have been recorded. However, as footprints tend to have rounded corners, pulse signals generated in the diagonal cable may not have been created at the tyres edges. Variations in vehicle load and tyre pressures can affect the shape of the tyre footprint.
For example, over inflation will decrease the footprint length at the edges and produce a more rounded footprint shape. It may, however, still be possible to use the measured widths to calculate the actual wheel path of the tyre. Dividing the measured tyre width in half and adding it to the tyres right hand side position measurement should produce an accurate wheel path position. Despite the fact that exact tyre widths could not be determined, significant width differences meant that it was possible to determine if an axle was a single tyre or double tyre assembly. Approximately 65% of all LGV axles were single tyre and 35% were double tyre. Survey samples for less common LGVs were too small to provide reliable data. Further measurement surveys are required to increase the data sample size.