Simple Function in python
def get_LOS(headway = None, frequency = None, veh_per_hr = None):


You have no docstring. That would helpful in explaining what the parameters are doing.

  if frequency != None and headway = veh_per_hr == None:


When checking wheter something is none it is best to use  is None or is not None

    headway = 1 / frequency


The problem is that if someone passes frequency along with one of the other parameters, this function will go merrily onwards and probably not even produce an error. I recommend having a get_LOS_from_frequency function which takes the frequency and then calls this function.

  if headway!= None or veh_per_hr != None:
    if headway 10 &lt; or veh_per_hr &gt; 6:


I'm pretty sure that won't compile

       return 'A'
    elif 10 &lt;= headway &lt; 15 or 5 &lt; veh_per_hr &lt;= 6:
      return 'B'
    elif 15 &lt;= headway &lt; 20 or 3 &lt; veh_per_hr &lt;= 5:
      return 'C'
    elif 20 &lt;= headway &lt; 31 or 2 &lt; veh_per_hr &lt;= 3:
      return 'D'
    elif 31 &lt;= headway &lt; 60 or 1 &lt;= veh_per_hr &lt; 2:
      return 'E'
    elif headway &gt;= 60 or veh_per_hr &lt; 1:
      return 'F'


I'd store these values in a list which makes it easy to pull them from configuration at a later date if neccesary.

    else:
      return 'Error'


Don't report error by returning strings. Throw an exception, or at least assert False.

How I'd do this:

import bisect
def get_los_from_frequency(frequency):
    return get_los(1 / frequency)

HEADWAY_LIMITS = [10, 15, 20, 31, 60]
VEH_PER_HR_LIMITS = [1,2,3,5,6]
GRADES = "ABCDEF"

def get_los(headway = None, veh_per_hr = None):
    if headway is None:
        headway_score = len(GRADES)
    else:
        headway_score = bisect.bisect_left(HEADWAY_LIMITS, headway)

    if veh_pr_hr is None:
        veh_pr_hr_score = len(GRADES)
    else:
        veh_pr_hr_score = len(GRADES) - bisect.bisect_left(VEH_PR_HR_LIMITS, veh_pr_hr)

    return GRADES[min(headway_score, veh_pr_hr_score)]