When a municipality seeks to ascertain the overall condition of their pavement network, there are often three primary factors that they consider: Surface Distress Index (SDI, or sometimes referred to as Pavement Condition Index), Roughness Index (RI) and Structural Index (SI). These factors can be combined in order to calculate an Overall Condition Index, or OCI.
While Surface Distress Index is vital and measures the length and severity of surface distresses, Structural Index and Roughness Index are considered enhanced attributes that can improve the reliability of a pavement condition assessment. Structural Index measures the strength of a pavement’s base, but Roughness is focused on something that all travelers will notice; just how bumpy is the street?
In engineering terms, roughness is measured with the International Roughness Index, or IRI. IRI is focused on measuring the change in elevation over a (relatively short) distance, expressed in millimeters/meter. You could also ask, is my road smooth?
When is Roughness Used?
Measuring roughness, often referred to as longitudinal profile, gives municipalities a better understanding of the condition of a particular road, but there are a few places where roughness may not provide the results you are looking for. Roughness that is calculated at extremely low speeds, or over manholes, speed bumps, railroad tracks and the like should be filtered out of a final OCI score. Failure to account for such roadway conditions will result in an inaccurate roughness score that adversely affects an otherwise pristine or newly paved street.
In a semi-automated pavement survey, roughness is measured via a series of gyroscopic sensors. These sensors detect minute changes in elevation over short distances, usually in millimeters per meter. Following a series of calculations that are performed within the pavement management system, a 0 -100 score is recorded. In common terms, a newer street would generally have a Roughness index above 85, while one due for an overlay would be in the range of 40-70. Failed streets typically have roughness values below 40. Some surface distresses such as raveling or rutting can have a large impact on the roughness readings, causing certain surface distresses to have a major impact on the Overall Condition Score once roughness has been included in the calculations.
When roughness is eventually implemented into the final OCI score, it generally accounts for 25-33% of the Overall Condition Score.
To Each Their Own
Though roughness can be a valuable factor in determining the overall condition of a roadway, not all agencies will choose to consider roughness in their final pavement condition score. Some pavement network conditions, such as brick crosswalks, cattle guards, railroad tracks and the like may discourage a municipality from trying to factor in roughness due to the complexities associated with filtering out pavement factors which ultimately crater the final roughness scoring. Additionally some popular pavement management programs, which have been in use by municipalities for years, are not able to factor in IRI readings into a final pavement condition score, leaving a lot of recorded data unused in these cases.
Ultimately roughness can be a very valuable factor to understanding the condition of a pavement network; however, agencies must take care when including roughness and ensure that it is not being used improperly. A meticulous quality control process should be implemented to confirm that roughness factors are not being considered inappropriately and roads are not being punished for their intended design.