If you are using table Trips (an employee of type driver
    makes between 0 and MANY trips using a company Truck);
   then when you have a new driver the list will be empty.
Should LimitToList = True then a new driver will never get her first trip.

-----------------------
Combo
ControlSource=tblTrips.EmployeeID
BoundColumn=1
RowSource=

    SELECT e1.EmployeeID, e1.Truck, 1
    FROM tblEmployees e1
    WHERE e1.Truck = [forms]![frmTrips]![TruckID]
UNION
    SELECT e2.EmployeeID, e2.Truck, 2
    FROM tblEmployees e2
    WHERE e2.Truck IS NOT NULL
      And e2.Truck <> [forms]![frmTrips]![TruckID]
UNION
    SELECT e3.EmployeeID, "", 3
    FROM tblEmployees e3
    WHERE e2.Truck IS NULL
ORDER BY 3, 1
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Select e1 employees who are set as the preferred driver for the truck,
   this can be no rows if no employees have the preferred the
       truck (ie new employee)
   this can be one row if only 1 employee has the truck
       preferred (normal)
   this can be more than one row when more than one
       driver has the same proffered truck.
UNION puts the selects together
Select e2 employees who are not set as the preferred driver for the truck
   but have a preferred truck.
Select e3 employees who are do not have a preferred truck.
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Bit of a mouthful - John