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.

It is conducted over multiple sites of uniform roughness. Each site should provide a range in roughness.

Each site must be surveyed multiple times at different speeds (typically every 10km/hr).

A Class 1 profiler is required to survey each site and this data is used as the Reference Data. This is collected at walking speed and typically requires lane closure.

The reference and survey data are plotted against each other and a coefficient is calculated at each speed. These coefficients are used to convert counts into IRI / NAASRA.

A new calibration is required if anything changed on the vehicle or there has been a significant change in weight in the vehicle.

The Lasers are Calibrated by the manufacture before leaving the workshop and require annual re-calibration.

Profiler Classification

Class III

Class I

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 BI only measures the response of the vehicle to the road roughness, in contrast to the laser profiler which directly measures the road profile.

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.
Data Accuracy

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 calibration of the response equipment is essential to ensure the calibration coefficients are correct.

Excessive movement in the vehicle, especially at low speed, can influence result accuracy.

The Class I laser profiler is the most accurate method for high speed, roughness data collection.
Survey Speed

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.

Braking/ Accelerating.

Changes to the vehicle that would affect its response to the road (e.g. wear or damage to vehicle suspension or even the weight inside).

Lasers are independent from the vehicle movement except in extreme conditions.

Excessive Braking/ Accelerating which cannot be compensated by the accelerometers.

Weather Restrictions

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.

Pavement Condition

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).