Table E-3. Level of Service Criteria for Un-signalized Intersections  Source: Highway Capacity Manual (Transportation Research Board, Special Report 209, 2000).  Based on EPA guidance (i.e., 40 CFR 93.123) intersections that are at a LOS of D, E, or F or that  will deteriorate to LOS D, E, or F with the  proposed action, should be modeled for CO  concentrations. Intersections that are at  LOS D, E, or F with a significant number of diesel  vehicles or that will change to LOS D, E, or F with the proposed action, should be modeled for  PM$_{10 }$and/or PM$_{2.5 }$concentrations.  Typically the top three critical intersections are selected based on the  abovementioned criteria  (i.e., significant number of diesel vehicles and worst LOS) and highest traffic volumes.  It is recommended to obtain project-specific data when co mputing a hot-spot analysis. These  data include peak traffic data (i.e., traffic volumes, traffic speeds, signal timing and type intersection control), and roadway configurations and geometry  (i.e., layout of intersections  delineating approaches and departures, number of lanes, roadway width, and number of turning  movements). For PM$_{10 }$ and PM$_{2.5 }$more detailed data (i.e., hourly traffic data) is necessary. This  91$_{ }$ E.2.3  Obtain Project Data  LOS  Average Control Delay  (seconds per vehicle)  Description  A  ≤10  Very low control delay 10 or less seconds per vehicle. All drivers find  freedom of operation. Very rarely more than one vehicle in queue.  B  >10 and ≤ 15  Control delay greater than 10 and up to 15 seconds per vehicle. Some  drivers begin to consider the delay troublesome. Seldom there is more  than one vehicle in queue.  C  >15 and ≤ 25  Control delay greater than 15 and up to 25 seconds per vehicle. Most  drivers feel restricted, but tolerably so. Often there is more than vehicle in queue.  D  >25 and ≤ 35  Control delay greater than 25 and up to 35 seconds per Drivers feel restricted. Most often, there is more than one vehicle in queue.  E  >35 and ≤ 50  Control delay greater than 35 and up to 50 seconds per Drivers find delays approaching intolerable levels. There is frequently  more than one vehicle in queue. This level denotes a state in which the  demand is close or equal to the probable maximum number of vehicles  that can be accommodated by the movement.  F  >50  Control delay in excess of 50 seconds per flow. Represents an intersection failure situation that is caused geometric and/or operational constraints external to the intersection.  Source: Highway Capacity Manual (Transportation Research Board, Special Report 209, 2000).  LOS  Average Control Delay  (seconds per vehicle)  Description  F  >80  Control delay greater than 80 seconds per vehicle; progression is very  poor, cycle lengths are long. Many individual cycle failures. flow rates exceed the capacity of the intersection. This considered unacceptable to most drivers. 