HttpSession is a key-object store. C# is strongly typed, you need to cast the result of the []-accessor.
TripAssignment[] logs = (TripAssignment[])HttpContext.Current.Session["DriverTripLog"];
TripAssignment log = logs[0];
By the way, you shouldnt use ArrayList, if possible.
ArrayList logs = (ArrayList)HttpContext.Current.Session["DriverTripLog"];
TripAssignment log = (TripAssignment)logs[0];