<PAGE>

EXHIBIT 10.20

                           ABM INDUSTRIES INCORPORATED
                     SUPPLEMENTAL EXECUTIVE RETIREMENT PLAN
                    PLAN DOCUMENT AS AMENDED DECEMBER 9, 2003

Preamble

This plan is an unfunded arrangement for a select group of management or
highly-compensated personnel of ABM Industries Incorporated (ABM) and its
subsidiaries. All rights under this Plan shall be governed by and construed in
accordance with the laws of the State of California.

ARTICLE I

Definitions

Section 1.01.

(a)      "ABM" means ABM Industries Incorporated, a Delaware corporation, its
         Subsidiaries and its corporate successors.

(b)      "Administrator" means the Controller of ABM or the person designated by
         the Committee with authority to manage and administer the operation of
         the Plan.

(c)      "Beneficiary" means the person, institution or trust designated by the
         Participant pursuant to 3.05 below to receive the Participant's
         interest in the Plan after the Participant's death.

(d)      "Committee" means the Compensation Committee of the Board of Directors
         of ABM Industries Incorporated.

(e)      "Fiscal Year" or "Year" (unless otherwise specified) means ABM's fiscal
         year as now constituted or as it may be changed hereafter from time to
         time.

(f)      "Participant" means an employee of ABM, or of a Subsidiary, designated
         by the Administrator for participation in the benefits of the Plan, or
         a person who was such at the time of his resignation, termination,
         retirement or death and who retains, or whose Beneficiaries obtain,
         benefits under the Plan in accordance with its terms.

(g)      "Payment Event" means a Participant's Retirement or in the event of
         earlier resignation, termination or death, the date the Participant
         attains or would have attained age 65.

(h)      "Plan" means this Supplemental Executive Retirement Plan as it may be
         amended from time to time.

(i)      "Retirement" means retirement at or after attaining age 65.

                                                                               1
<PAGE>

EXHIBIT 10.20

(j)      "Supplemental Benefit" means the total amount allocated to the benefit
         of a Participant under the Plan.

(k)      "Subsidiary" means a company of which ABM owns, directly or indirectly,
         at least a majority of the shares  having  voting power in the election
         of directors.

ARTICLE II

Designation of Participants and Allocation of Total Fund

Section 2.01. The Administrator shall at least once in each Fiscal Year
irrevocably specify:

(a)      The name of each employee who shall be entitled to participate in the
         Plan for such Year; and

(b)      The amount to be allocated for the benefit of each Participant for such
         Year.

Effective December 31, 2002, there shall be no new Participants in the Plan and
designations by the Administrator shall be limited to allocations to active
employees of ABM who are participants in the Plan.

Section 2.02. The amount to be allocated for the benefit of each Participant
shall be determined in accordance with the terms of the most recent Grant
Certificate in existence for each Participant, which Grant Certificates have
been approved by the Committee. The Administrator shall report to the Committee
the amounts allocated and Participants for such Year.

ARTICLE III

Future Payments

Section 3.01. The Administrator shall cause an accrual account to be kept in the
name of each Participant and each Beneficiary of a deceased Participant. The
accrual account shall reflect the value of the Supplemental Benefits payable to
such Participant or Beneficiary under the Plan.

Section 3.02. Until and except to the extent that Supplemental Benefits
hereunder are distributed to the Participants or Beneficiaries from time to time
in accordance with orders of the Administrator, the interest of each Participant
and Beneficiary herein is that of a general creditor of ABM and is contingent on
and subject to forfeiture as provided in Section 3.06. Title to and beneficial
ownership of any assets, whether cash or investments, which ABM may set aside or
accrue to meet its obligations hereunder, shall at all times remain the property
of ABM. No Participant or Beneficiary shall under any circumstances acquire any
property interest in any specific assets of ABM.

                                                                               2
<PAGE>

EXHIBIT 10.20

Section 3.03. Upon resignation, termination, Retirement or death of a
Participant, the value of the Supplemental Benefits payable to such Participant
or Beneficiary shall be determined with reference to the accrual account
maintained for such Participant.

Section 3.04. Payment of the amount allocated to a Participant shall be deferred
until the occurrence of a Payment Event. If the Participant dies before
receiving any or all of the payments due the Participant, any remaining amount
shall be paid, but not before the Participant would have reached age 65, to the
Beneficiary. After determining the value of the Supplemental Benefit for a
Participant entitled to payment, the Administrator shall arrange to pay 1/120th
of the value of the account to the Participant or Beneficiary each month for a
period of 10 years from the date of the Payment Event. There shall be no gains
or losses allocated to the account during the 10 year period of payment.

Section 3.05. Each Participant shall have the right to designate a Beneficiary
or Beneficiaries who are to succeed to his right to receive future payments
hereunder in the event of his death. In case of a failure of designation or the
death of a designated Beneficiary without a designated successor, distribution
shall be made to the Participant's estate or trust, if a trust for such purpose
is in existence. No designation of Beneficiary shall be valid unless in writing
signed by the Participant, dated, and filed with the Committee. Beneficiaries
may be changed without consent of any prior Beneficiaries.

Section 3.06. The right of a Participant or Beneficiary to receive future
payments hereunder shall be vested at all times; provided, however, that such
right shall be forfeited immediately upon the occurrence of either of the
following events: If the Participant is discharged from employment by ABM or a
subsidiary for acts which, in the opinion of the ABM, constitute embezzlement of
corporate funds or if, following the Participant's termination of employment, it
is determined that he or she has embezzled corporate funds.

Section 3.07. Nothing contained herein shall be deemed to create a trust of any
kind for the benefit of any Participant or Beneficiary, or create any fiduciary
relationship. Funds accrued hereunder shall continue for all purposes to be a
part of the general funds of ABM, and no person other than ABM shall, by virtue
of the provisions of this Plan, have any interest in such funds. To the extent
that any person acquires a right to receive payments from ABM under this Plan,
such right shall be no greater than the right of any unsecured general creditor
of ABM.

Section 3.08. The adoption of this Plan shall not confer upon any employee of
ABM or any of its subsidiaries or Participant any right to continued employment,
nor shall it interfere in any way with the right of ABM or any of its
Subsidiaries to terminate the employment or change the compensation of any of
its employees at any time.

                                                                               3
<PAGE>

EXHIBIT 10.20

ARTICLE IV

Administration

Section 4.01. The books and records to be maintained for the purpose of the Plan
shall be maintained by the officers and employees of ABM at its expense and
subject to the supervision and control of the Administrator. ABM shall pay all
expenses of administering the Plan.

Section 4.02. To the extent permitted by law, the right of any Participant or
any Beneficiary in any benefit or to any payment hereunder shall not be subject
in any manner to attachment or other legal process for the debts of such
Participant or Beneficiary; and any such benefit or payment shall not be subject
to anticipation, alienation, sale, transfer, assignment or encumbrance. In the
event that the Plan is presented with an appropriate order from a family court
or other court of competent jurisdiction dividing the right to benefits under
this Plan or to receiving continuing payments under this Plan between the
Participant and the Participant's spouse, the Administrator shall establish such
accounts and sub-accounts and make arrangement for such payments as the order
may require. In no event shall the Plan be required to pay a benefit in a
greater amount or earlier than would otherwise be required by the Plan for
payments to the Participant.

Section 4.03. No member of the Committee or the Administrator and no officer or
employee of ABM shall be liable to any person for any action taken or omitted in
connection with the administration of this Plan unless attributable to his own
fraud or willful misconduct; nor shall ABM be liable to any person for any such
action unless attributable to fraud or willful misconduct on the part of a
director, officer or employee of ABM.

Section 4.04. The Committee shall establish procedures for handling claims for
benefits under the Plan and appeals from denied claims.

ARTICLE V

Amendment of Plan

Section 5.01. The Committee may amend the Plan in whole or in part from time to
time.

Section 5.02. Notice of every such amendment shall be given in writing to each
Participant and Beneficiary of each deceased Participant.

ARTICLE VI

Entire Agreement

This Supplemental Executive Retirement Plan Document and the most recently dated
Grant Certificate delivered to a Participant and properly signed by an officer
of ABM, shall supersede all prior plans, documents, agreements, offers,
contracts or clauses,

                                                                               4
<PAGE>

EXHIBIT 10.20

whether designated as "Executive Retirement", "Post Employment Consultancy" or
by any other term, which refer to the benefit of such Participant which is the
subject matter of this Plan Document.

Encls: SERP Grant Certificate
       SERP Designation of Beneficiary Form

                                                                               5
<PAGE>

EXHIBIT 10.20

ABM Industries Incorporated
SUPPLEMENTAL EXECUTIVE RETIREMENT PLAN
DESIGNATION OF BENEFICIARY

It is important that each Participant in the Company's Supplemental Executive
Retirement Plan (SERP) designate a beneficiary for the payment of Plan benefits
in the event of the Participant's death prior to a full distribution of
benefits.

Please return a copy of this completed Designation of Beneficiary form to ABM
Industries, Attn: Executive Compensation Administrator, 160 Pacific Avenue, San
Francisco, California 94111.

This form should be promptly updated by the Participant whenever there is a
change of address or designated beneficiary.

SECTION I:  PERSONAL INFORMATION

Name____________________________Spouse's Name__________________________________

SSN:____________________________Company/Location_______________________________

Home Address____________________City/State/Zip_________________________________

SECTION II: DESIGNATION OF BENEFICIARY

Pursuant to the terms and conditions of the Plan, I hereby designate the
following as my beneficiary(ies), to whom any benefits I may then have in the
Plan may be paid upon my death. This designation supersedes any prior
beneficiary designation made by me with respect to these benefits.

Primary: I name the following person(s) or entity(ies) as my Primary
Beneficiary(ies):

Name:_____________________________________SSN__________________________________

Address_________________________________________________________________________

Name:_____________________________________SSN___________________________________

Address_________________________________________________________________________

Secondary: If my Primary beneficiary(ies) is (are) unable to receive this
distribution, I Name the following Secondary person(s) or entity(ies) as my
Secondary Beneficiary(ies):

Name_____________________________________SSN___________________________________

Address________________________________________________________________________

Name______________________________________SSN__________________________________

Address________________________________________________________________________


I HEREBY CERTIFY THAT THE ABOVE INFORMATION IS TRUE AND CORRECT. THE COMPANY,
PLAN ADMINISTRATOR AND ANY OTHER PERSONS ASSOCIATED WITH THE ADMINISTRATION OF
THE PLAN ARE ENTITLED TO RELY ON THIS DOCUMENT AND SHALL BE FREE OF LIABILITY
FOR ANY ACTION TAKEN UNDER THE PROVISIONS OF THE PLAN AND IN RELIANCE ON THIS
DOCUMENT.

________________________________________      __________________________________
Participant's Signature/Date                  Spouse's Signature/Date

Witness to Signatures:__________________________________________________________

                                                                               6