Roughness – What are you getting?
99% of roughness surveys, in New Zealand, are conducted by utilizing either a Response Type Roughness Measurement System (RTRMS) like our Bump Integrator (BI) or point laser equipment. While they both provide roughness results, there are differences in the technology which can affect the accuracy of the data provided by the supplier.
Some things like the validation process, needing a distance measuring device and how you conduct the surveys are the same; other parts differ.
What is the Same:
- Validations are required prior to survey to ensure the equipment is operating correctly and there is no bias error. Typically, New Zealand abides by Austroads Test Methods which require initial baseline data required for repeatability and on-going monitoring providing a bias error.
- Both systems require a Distance Measuring Instrument (DMI) which will need to be calibrated on a regular basis.
- Both require smooth driving – I like to call it Nana driving! Erratic driving, braking / accelerating can influence the results (typically increases the roughness).
- As and when braking / accelerating cannot be avoided, or surface events are occurred, these can be recorded in the data.
- Both systems can deliver NAASRA / roughness data at 20m & 100m intervals as well as 100m rolling averages.
What is Different:
|Bump Integrator (BI)||Point Lasers|
Calibration is required on a frequent basis to ensure a change in the vehicle response (with age/km) doesn’t affect data accuracy.
The Lasers are Calibrated by the manufacture before leaving the workshop and require annual re-calibration.
World Bank Classification - Class I – V decrease with accuracy and reliability.
|How is the IRI calculated?|
As the vehicle moves along the pavement, it records the number of times (counts) that the BI moves in a vertical direction over a certain distance. This information is then converted into Lane IRI or NAASRA utilising the coefficients calculated in the Calibration process.
|The two lasers directly record the road profile long both wheel paths. These are coupled with accelerometers which record vehicle movement (not related to the road profile). These profiles, combined, provide the true surface profile and are then converted into IRI and / or NAASRA.|
The data recorded by the BI can general be as accurate as the point lasers in the range of 2.5 to 6 IRI (approx. 65 to 155 NAASRA) – however it comes down to accuracy of calibration and validation.
|The Class I laser profiler is the most accurate method for high speed, roughness data collection.|
Can survey as slow as 10 km/h and as fast is 100 km/h, if the equipment calibration went as low / up to those speeds.
Typically, a survey can go as slow as 25 km/h before the data is compromised. Maximum speed is 100 km/h.
|Factors that Influence the Results|
Excessive movement in the vehicle especially at low speed can influence the number of counts.
Lasers are independent from the vehicle movement except in extreme conditions.
Can survey when the pavement surface is wet. However, some vehicle springs behave differently when it is hot or cold and wet. In that case, some vehicles will need to be calibrated under both conditions.
Cannot survey when the pavement is wet.
Response equipment operate well between an IRI of 2.5 and 8 (approx. 65 to 210 NAASRA). It can operate along roads which have a higher IRI however there is more risk of damaging the equipment.
Lasers operate best between an IRI of 1.5 and 6 (approx. 35 to 155 NAASRA).