def is_single_unit_truck(df):
    """Return if vehicle is a single unit truck, per NHTSA convention."""
    yr = df['YEAR']
    body = df['BODY_TYP']
    tow_veh = df['TOW_VEH']
    return yr.between(1975, 1981) & body.isin([53, 54, 55, 56, 60]) & (tow_veh
         == 0) | yr.between(1982, 1990) & body.isin([70, 71, 72, 75, 76, 78,
        79]) & tow_veh.isin([0, 9]) | (1991 <= yr) & body.isin([60, 61, 62,
        63, 64, 67, 71, 72, 78, 79]) & tow_veh.isin([0, 5, 6, 9])
