A traffic rated Packaged Metering Manhole integrating a 3-inch Parshall flume to measure industrial discharge.  The traffic reducer at the top is a 38-inch diameter model - most manhole openings are 22 1/2-inches or 31 3/4-inches. The larger opening makes it easier for the operator to enter / exit the manhole (at the cost of making the lid heavier and harder to removed).  

For large opening traffic reducer tops, Openchannelflow suggests using lightweight composite covers from GMI Composites (a division of East Joran Ironworks) or Fiberlite.