There are two key criteria for what we regard as defects requiring immediate attention on the highway.
In the carriageway a depth of 50mm or greater would result in the defect being marked for repair.
In the footway a depth/height of 20mm or greater would result in the defect being marked for repair.
The roads in Slough are inspected on a frequency commensurate with their assessed importance within the overall highway network. This means that some residential roads are inspected only twice in any 12 month period. Because of this defects that do not meet the criteria above may be marked for repair as in the inspector's opinion they are likely to reach 'intervention level' before the next inspection.
In addition the criteria are not absolute. Each inspector needs to consider the location of the defect, the volume of traffic, the nature of the traffic etc., usage by children, elderly and disabled persons and the extent of visibility at the site. Also if a road is due to be resurfaced in the near future there is less likelihood that defects that have yet to reach criteria will be repaired.
There is a degree of personal judgement in highway inspection and inspectors are encouraged to err on the side of caution.
In terms of measuring each inspector carries a metal tape. For ease of reference a 20p coin is just over 20mm side to side.
I intend to review the highway inspection process some time in the new financial year following the publication of the revised LAA Code of Good Practice for Highway Maintenance. However as this document has not yet been published I cannot say exactly when the review will be complete.
I hope this helps but if you would like any further information about this subject please contact me.
Slough Borough Council