<PAGE>

                                                                    Exhibit 4.16
                               SIXTH AMENDMENT TO
                                CREDIT AGREEMENT
                                       and
                                     CONSENT

                  THIS SIXTH AMENDMENT TO CREDIT AGREEMENT and CONSENT (the
"Amendment") is dated as of September 30, 2001 by and among APCOA/STANDARD
 ---------
PARKING, INC., a Delaware corporation (the "Company"), the financial
                                            -------
institutions listed on the signature pages hereof (the "Lenders"), and BANK ONE,
                                                        -------
NA (having its principal place of business in Chicago, Illinois), in its
individual capacity as a Lender and in its capacity as contractual
representative (the "Agent") under that certain Credit Agreement among the
                     -----
Company, the lenders party thereto and the Agent dated as of March 30, 1998 (as
clarified by letter agreement dated March 30, 1999 and by letter agreement dated
August 23, 2000, and as amended by a First Amendment to Credit Agreement dated
as of November 12, 1999, a Second Amendment to Credit Agreement dated as of
March 30, 2000, a Third Amendment to Credit Agreement dated as of May 12, 2000,
a Fourth Amendment to Credit Agreement dated as of November 14, 2000 and a Fifth
Amendment to Credit Agreement dated as of March 30, 2001, the "Credit
                                                               ------
Agreement"). Defined terms used herein and not otherwise defined herein shall
---------
have the meaning given to them in the Credit Agreement.

                                   WITNESSETH

                  WHEREAS, the Company, the Lenders and the Agent are parties to
the Credit Agreement;

                  WHEREAS, the Company has requested that the Required Lenders
agree to certain amendments to the Credit Agreement and to consent to the
"Specified Transaction" (as defined in Section 2 below);
                                       ---------

                  WHEREAS, the Company, the Lenders and the Agent have agreed to
enter into this Amendment on the terms and conditions set forth herein, to amend
the Credit Agreement and to consent to the Specified Transaction in the manner
hereinafter set forth;

                  NOW, THEREFORE, in consideration of the premises set forth
above, the terms and conditions contained herein, and other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged, the
parties hereto have agreed to the following amendment to and consent under the
Credit Agreement:

                  1. Amendments to the Credit Agreement. Effective as of the
                     ----------------------------------
date hereof and subject to the satisfaction of the conditions precedent set
forth in Section 3 below, the Credit Agreement is hereby amended as follows:
         ---------

<PAGE>

                  (a) The Definition of "Adjusted EBITDA" in Section 1.1 of the
                                         ---------------     -----------
Credit Agreement shall be amended to renumber the existing clause (I)(xii) as
                                                           --------------
clause (I)(xiii) and to insert the following new clause (I)(xii) immediately
----------------                                 ---------------
prior thereto:

                  (xii)  solely with respect to the fiscal quarter ended
             September 30, 2001, restructuring and special charges,

                  (b) Section 5.2(a) of the Credit Agreement is hereby deleted
                      --------------
in its entirety and the following new Sections 5.2(a) is substituted therefor:
                                      ---------------

                  (a) Adjusted Total Debt to Adjusted EBITDA Ratio. Permit or
                      --------------------------------------------
         suffer the Adjusted Total Debt to Adjusted EBITDA Ratio to be greater
         than (i) 6.95 to 1.0 at any time from and including the Effective Date
         to and including September 29, 1999, (ii) 6.75 to 1.0 at any time from
         and including September 30, 1999 to and including December 31, 1999,
         (iii) 8.15 to 1.0 at any time from and including January 1, 2000 to and
         including September 30, 2000, (iv) 7.99 to 1.0 at any time from and
         including October 1, 2000 to and including December 31, 2000, (v) 8.23
         to 1.0 at any time from and including January 1, 2001 to and including
         March 31, 2001, (vi) 8.07 to 1.0 at any time from and including April
         1, 2001 to and including June 30, 2001, (vii) 7.77 to 1.0 at any time
         from and including July 1, 2001 to and including September 30, 2001,
         (viii) 6.54 to 1.0 at any time from and including October 1, 2001 to
         and including December 31, 2001, (v) 6.74 to 1.0 at any time from and
         including January 1, 2002 to and including March 31, 2002 and (vi) 6.42
         to 1.0 at any time from and including April 1, 2002 to and including
         June 30, 2002.

                  2. Consent. Notwithstanding the provisions of Section 5.2(i)
                     -------                                    --------------
of the Credit Agreement to the contrary, effective as of the date hereof and
subject to the satisfaction of the conditions precedent set forth in Section 3
                                                                     ---------
below, the Agent and the Lenders hereby agree to permit the Company to eliminate
the $8,000,000 intercompany account balance presently owing by the Parent in
favor of the Company pursuant to a two-step, non-cash transaction consummated
during the fiscal quarter ending December 31, 2001 whereby (a) the Company shall
pay the Parent $8,000,000 to redeem $8,000,000 of its Preferred Stock presently
owned by the Parent and (b) the Parent shall use the proceeds of the payment
received under clause (a) to contemporaneously pay the Company $8,000,000 in
               ----------
full satisfaction of the aforementioned intercompany account balance
(collectively, the "Specified Transaction"); provided, however, that both before
                                             --------  -------
and upon consummation of the Specified Transaction, (i) no Default or Unmatured
Default shall have occurred or be continuing and (ii) the representations and
warranties contained the Loan Documents shall be true and correct.

                  3. Conditions of Effectiveness. The effectiveness of this
                     ---------------------------
Amendment is subject to the condition precedent that the Agent shall have
received:

                  (a) duly executed originals of this Amendment from each of the
         Company, the Required Lenders and the Agent;

                  (b) duly executed originals of a Reaffirmation in the form of
         Exhibit A attached hereto from the Guarantors; and


                                       2

<PAGE>

                  (c) an amendment fee of $100,000, payable to the Agent for the
         pro rata benefit of the Lenders based upon their respective
         Commitments.

                  4. Representations and Warranties of the Company. The Company
                     ---------------------------------------------
hereby represents and warrants as follows:


                  (a) The Company has the corporate power and authority to
execute and deliver this Amendment and the officers of the Company executing
this Amendment have been duly authorized to execute and deliver the same and
bind the Company with respect to the provisions hereof.

                  (b) This Amendment and the Credit Agreement as previously
executed and amended and as amended hereby, constitute legal, valid and binding
obligations of the Company and are enforceable against the Company in accordance
with their terms (except as enforceability may be limited by bankruptcy,
insolvency, or similar laws affecting the enforcement of creditors' rights
generally).

                  (c) Upon the effectiveness of this Amendment and after giving
effect hereto, (i) the Company hereby reaffirms all covenants, representations
and warranties made in the Credit Agreement as previously amended and as amended
hereby, and agrees that all such covenants, representations and warranties
(other than covenants, representations and warranties that are expressly made as
of a specific date) shall be deemed to have been remade as of the effective date
of this Amendment and (ii) no Event of Default or Unmatured Event has occurred
and is continuing.

                  5. Reference to and Effect on the Credit Agreement.
                     ------------------------------------------------

                  (a) Upon the effectiveness of Section 1 hereof, on and after
the date hereof, each reference in the Credit Agreement or in any other Loan
Document (including any reference therein to "this Credit Agreement," "this
Agreement," "hereunder," "hereof," "herein" or words of like import referring
thereto) shall mean and be a reference to the Credit Agreement as amended
hereby.

                  (b) Except as specifically amended or consented to above, the
Credit Agreement and all other documents, instruments and agreements executed
and/or delivered in connection therewith, shall remain in full force and effect,
and are hereby ratified and confirmed. The amendments set forth herein shall be
limited precisely as provided for herein, and shall not be deemed to be a waiver
of, amendment of, consent to or modification of any other term, provision or
Event of Default or Unmatured Event of or under the Credit Agreement or of any
term or provision of any other Credit Document or of any transaction or further
or future action on the part of the Company which would require the consent of
the Agent or any Lender under the Credit Agreement.

                  (c) The execution, delivery and effectiveness of this
Amendment shall not, except as expressly provided herein, operate as a waiver of
any right, power or remedy of the Agent or the Lenders, nor constitute a waiver
of any provision of the Credit Agreement or any other documents, instruments and
agreements executed and/or delivered in connection therewith.

                                       3

<PAGE>

                  6. Costs and Expenses. The Borrower agrees to pay all
                     ------------------
reasonable costs, fees and out-of-pocket expenses (including attorneys' fees and
expenses of Sidley Austin Brown & Wood, special counsel to the Agent) incurred
by the Agent in connection with the preparation, arrangement, execution and
enforcement of this Amendment.

                  7. GOVERNING LAW. THIS AMENDMENT SHALL BE GOVERNED BY AND
                     -------------
CONSTRUED IN ACCORDANCE WITH THE INTERNAL LAWS (INCLUDING 735 ILCS 105/5-1 ET
SEQ., BUT OTHERWISE WITHOUT REGARD TO THE CONFLICT OF LAW PROVISIONS) OF THE
STATE OF ILLINOIS.

                  8. Headings. Section headings in this Amendment are included
                     --------
herein for convenience of reference only and shall not constitute a part of this
Amendment for any other purpose.

                  9. Counterparts. This Amendment may be executed by one or more
                     ------------
of the parties to the Amendment on any number of separate counterparts and all
of said counterparts taken together shall be deemed to constitute one and the
same instrument.

                                       4


<PAGE>

                  IN WITNESS WHEREOF, this Amendment has been duly executed as
of the day and year first above written.

                                 APCOA/STANDARD PARKING, INC., as the Company


                                 By: /s/ G. Marc Baumann
                                     ---------------------------
                                 Name:   G. Marc Baumann
                                 Title:  Executive Vice President
                                         Chief Financial Officer and Treasurer


                                 BANK ONE, NA (Main Office Chicago) (formerly
                                 known as THE FIRST NATIONAL BANK OF
                                 CHICAGO), as Agent and as a Lender

                                 By: /s/ Thomas T. Bower
                                     ---------------------------
                                 Name:   Thomas T. Bower
                                 Title:  Senior Vice President


                                 LASALLE BANK NATIONAL ASSOCIATION
                                 (formerly known as LaSalle National Bank), as
                                 a Lender


                                 Bt: /s/ Sean P. Silver
                                     ---------------------------
                                 Name:   Sean P. Silver
                                 Title:  Vice President




                      Signature Page to Sixth Amendment to
                          APCOA/STANDARD PARKING, INC.
                          Credit Agreement and Consent

<PAGE>

                                    EXHIBIT A
                                    ---------
                                  REAFFIRMATION

                  Each of the undersigned hereby acknowledges receipt of a copy
of the foregoing Sixth Amendment to Credit Agreement and Consent dated as of
September 30, 2001 (the "Amendment") by and among APCOA/STANDARD PARKING, INC.,
                         ---------
a Delaware corporation (the "Company"), the financial institutions from time to
                             -------
time party thereto (the "Lenders"), and Bank One, NA, in its individual capacity
                         -------
as a Lender and in its capacity as contractual representative (the "Agent"),
which Amendment further amends that certain Credit Agreement among the Company,
the lenders party thereto and the Agent dated as of March 30, 1998 (as clarified
by letter agreement dated March 30, 1999 and by letter agreement dated August
23, 2000, and as amended by a First Amendment to Credit Agreement dated as of
November 12, 1999, a Second Amendment to Credit Agreement dated as of March 30,
2000, a Third Amendment to Credit Agreement dated as of May 12, 2000, a Fourth
Amendment to Credit Agreement dated as of November 14, 2000 and a Fifth
Amendment to Credit Agreement dated as of March 30, 2001, the "Credit
Agreement"). Capitalized terms used in this Reaffirmation and not defined herein
shall have the meanings given to them in the Credit Agreement.

                  Without in any way establishing a course of dealing by the
Agent or any Lender, each of the undersigned reaffirms the terms and conditions
of the Guaranty and any other Loan Document executed by it and acknowledges and
agrees that such agreements and each and every such Loan Document executed by
the undersigned in connection with the Credit Agreement remains in full force
and effect and is hereby reaffirmed, ratified and confirmed. All references to
the Credit Agreement contained in the above-referenced documents shall be a
reference to the Credit Agreement as so modified by the Amendment and as the
same may from time to time hereafter be amended, modified or restated.

                      REMAINDER OF PAGE INTENTIONALLY BLANK

<PAGE>

                  IN WITNESS WHEREOF, each of the undersigned has caused its
duly authorized officer to execute this Reaffirmation as of the 30th day of
September, 2001.

                                 A-1 Auto Park, Inc.

                                 By: /s/  G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Vice President, Treasurer

                                 AP Holdings, Inc.

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Treasurer

                                 APCOA Capital Corporation

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Vice President, Treasurer

                                 Century Parking, Inc.

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Vice President, Treasurer

                                 Events Parking Co., Inc.

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Treasurer

                                 Hawaii Parking Maintenance, Inc.

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Vice President, Treasurer


                        SIGNATURE PAGE TO REAFFIRMATION
                         (Sixth Amendment and Consent)

<PAGE>

                                 Metropolitan Parking System, Inc.

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Treasurer

                                 S & S Parking, Inc.

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Vice President, Treasurer

                                 Sentinel Parking Co. of Ohio, Inc.

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Vice President, Treasurer

                                 Sentry Parking Corporation

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Vice President, Treasurer

                                 Standard Auto Park, Inc.

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Treasurer

                                 Standard Parking Corporation

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Treasurer


                        SIGNATURE PAGE TO REAFFIRMATION
                         (Sixth Amendment and Consent)



<PAGE>

                                 Standard Parking Corporation IL

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Treasurer

                                 Tower Parking, Inc.

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Vice President, Treasurer

                                 Virginia Parking Service, Inc.

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Vice President, Treasurer

                                 APCOA Bradley Parking Company, LLC
                                 By:  APCOA/Standard Parking, Inc., its
                                      Sole Member

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Executive Vice President, Chief Financial
                                     Officer, Treasurer


                                 APCOA LaSalle Parking Company, L.L.C.
                                 By:  APCOA/Standard Parking, Inc., its Manager

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Executive Vice President, Chief Financial
                                     Officer, Treasurer




                        SIGNATURE PAGE TO REAFFIRMATION
                         (Sixth Amendment and Consent)



<PAGE>

                                 Executive Parking Industries, L.L.C.
                                 By:  APCOA/Standard Parking, Inc., its Manager

                                 By: /s/ G. Marc Baumann
                                    --------------------------------------------
                                     G. Marc Baumann
                                     Executive Vice President, Chief Financial
                                     Officer, Treasurer


                        SIGNATURE PAGE TO REAFFIRMATION
                         (Sixth Amendment and Consent)