Explanation of In Tunnel Data

Carbon monoxide is produced by vehicles and emitted via their tailpipes. Carbon monoxide levels inside the tunnel are closely monitored to ensure that air quality within the tunnel is within acceptable Limits.

Limit refers to the maximum allowable in-tunnel carbon monoxide Single Point or Exposure level as imposed on the Cross City Tunnel’s operation under the Planning Minister’s Conditions of Approval.

Exposure values are calculated over 15 and 30 minute averaging periods so that a direct comparison can be made against the Limit.

The Single Point and Exposure values shown in the table relate to carbon monoxide levels in the tunnel. They are all measured in ppm or “parts per million”. A percentage value is also shown in brackets which is the value shown as a percentage of the allowable Limit – so a reading greater than 100% would indicate a value over the Limit.

Single Point values refer to the concentration of carbon monoxide measured at any of the monitors mounted on the tunnel walls along the length of the tunnel. Single Point values are measured over a 3 minute averaging period (a short averaging period) to measure peak readings.

Exposure values are a computed estimate of the carbon monoxide levels that a motorist would experience whilst travelling through the tunnel, as opposed to simply the levels of carbon monoxide present in the tunnel (Single Point values). Exposure to carbon monoxide is dependent upon the time exposed as well as the concentrations of carbon monoxide.  Exposure is therefore calculated from the Single Point values and the journey time through the tunnel.

15 minute rolling average (ppm) – This refers to the Exposure to carbon monoxide which a motorist may have received whilst travelling through the tunnel averaged over 15 minutes.   The maximum for any route (eastbound or westbound) is displayed. The average is computed from the levels measured during the time in the tunnel (up to 15-minutes) and the remainder (if any) of the 15 minutes outside the tunnel where the carbon monoxide level is assumed for this purpose to be zero.   Rolling averages are frequently updated and refer to the previous 15 minutes.

30 minute rolling average (ppm) – This refers to the maximum Exposure to carbon monoxide which a motorist may have received whilst travelling through the tunnel averaged over 30 minutes.  The same method of averaging is used as for the 15 minute averages.

The Single Point and Exposure carbon monoxide values shown are the levels recorded during the preceding 1-hour period. In reality the carbon monoxide levels are being constantly recorded, however for practical reasons the information currently shown in the table relates only to the maximum values measured over the 1-hour shown.