Sequential Numbering based on last numbering

[CCWeedcontrol]

I have a road layer that i need to add sequential numbering to, the RD_ID field. Some already have a RD_ID number and some are missing. I need to populate attributes of only the missing ones that only have CC in front of them but in sequential order but starting from the last number left off at. I attached a picture of what i need. thanks.

I have the following that adds sequential ID but it doesn't take in consideration where the last RD_ID left off.

import arcpy

fc = r'C:\Temp\Roads.shp'

startNumber = 0

with arcpy.da.UpdateCursor(fc, "RD_ID") as cursor:
    for row in cursor:
            row[0] = startNumber
            startNumber = startNumber + 1
            cursor.updateRow(row)

print 'Done'‍‍‍‍‍‍‍‍‍‍‍‍‍

[CCWeedcontrol]

I add an if statement but the null and blanks one don't get updated sequentially. I get no error's, i did add "=" to line 26.

RD_ID += 1

import arcpy

fc = r'C:\Temp\Address_Points.shp'

#arcpy.CalculateField_management(fc, "RD_ID", '""" """', "PYTHON")

rd_list = []          
with arcpy.da.SearchCursor(fc, ["RD_ID"]) as cursor:          
    for row in cursor:  
        try:          
            if "CC" in row[0]:                
                rd_list.append(int(row[0].strip("CC")))             
        except TypeError:          
            pass                  
del cursor          
 
              
rd_list.sort()          
RD_ID = rd_list[-1] + 1          
    
#whereclause = "RD_ID = '%CC%'"
with arcpy.da.UpdateCursor(fc, "RD_ID") as rows:           
    for row in rows:
        if row[0] in (""," ", None):
            row[0] = 'CC%05d' %RD_ID 
            RD_ID += 1                      
            rows.updateRow(row)      
    del row           


print 'Done'    
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[forestknutsen1]

They are not getting updated at all? Or they are not getting the right values when updated? Can you set some breakpoints around the if statement and see the behavior?