<PAGE>   1

                                                                   Exhibit 10.13

                          MUTUAL SEPARATION AND RELEASE

         This Mutual Separation and Release ("Agreement") is by and among Gordon
Quick and HighwayMaster Corporation, its employees, officers, directors, agents,
predecessors, successors, parents, subsidiaries, affiliates, and all related
entities including HighwayMaster Communications, Inc. (individually and
collectively referred to as "HighwayMaster"). The parties agree as follows:

1.       In accordance with the HighwayMaster's regular payroll practices,
         HighwayMaster shall pay Mr. Quick $30,502.10 per month for the period
         from the date of this Agreement through January 15, 1999, and
         $33,552.30 per month for the period from January 16, 1999 through April
         30, 1999. On May 1, 1999, HighwayMaster shall pay Mr. Quick a cash
         payment of $434,797, plus $14,193 if Mr. Quick shall have provided at
         least 80 hours of service to HighwayMaster during the period from
         January 1, 1999 through January 31, 1999, plus $26,296 if Mr. Quick
         shall have provided at least 80 hours of service to HighwayMaster
         during the period from February 1, 1999 through February 28, 1999, plus
         $13,000 if Mr. Quick shall have worked at least 80 hours of service to
         HighwayMaster during the period from March 1, 1999 through March 31,
         1999, plus $6,200 if Mr. Quick shall have worked at least 80 hours of
         service to HighwayMaster during the period from April 1, 1999 through
         April 30, 1999, less withholding and other deductions as required by
         law.

2.       Except as specified in this Agreement and company policy, all other
         fringe benefits and perquisites related to Mr. Quick's employment will
         end as of April 30, 1999, which is the day his employment ends.
         HighwayMaster and Mr. Quick acknowledge that Mr. Quick has received
         concurrently with, or prior to, the execution of this Agreement, all
         amounts payable by HighwayMaster for reimbursement of expense to the
         date of this Agreement, all amounts payable by HighwayMaster for
         reimbursement of expense to the date of this Agreement, unused vacation
         days less any required withholding and other appropriate deductions
         (totaling 40 hours of accrued vacation), and any other amounts payable
         or reimbursements due to Mr. Quick by HighwayMaster for any reason to
         the date of this Agreement.

3.       HighwayMaster and Mr. Quick acknowledge the sufficiency of the
         consideration exchanged hereunder in connection with each of their
         promises herein.

4.       Commencing from the date of this Agreement until April 30, 1999, Mr.
         Quick will have the duties and responsibilities of chief Operating
         Officer of HighwayMaster as determined by the Chief Executive Officer
         of HighwayMaster, but in no event shall such duties and
         responsibilities obligate Mr. Quick to perform services for
         HighwayMaster in excess of 80 hours in each of January 1999 and
         February 1999. The parties understand and agree that Mr. 



                                      -1-
<PAGE>   2

         Quick may seek and secure alternative employment at any time during
         this period and that to the extent Mr. Quick does not provide at least
         80 hours of service in each of January 1999 and February 1999,
         HighwayMaster's only recourse is that it should not be obligated to
         make the additional incremental payments of $14,193.00, $26,296.00,
         $13,000.00 and $6,200.00 as specified in Section 1 hereof. Mr. Quick
         agrees that he will relinquish the title and role of Chief Operating
         Officer of HighwayMaster Corporation and HighwayMaster Communications,
         Inc. effective on April 30, 1999.

5.       After termination of his employment, Mr. Quick agrees to provide
         HighwayMaster an additional 40 hours for litigation consultation as
         reasonably requested upon reasonable notice by HighwayMaster after
         April 30, 1999. If HighwayMaster requests Mr. Quick to engage in travel
         in connection with providing such services, HighwayMaster will
         reimburse Mr. Quick's reasonable travel and lodging expenses in
         accordance with its standard travel reimbursement policies.

6.       Mr. Quick understands and agrees that all of the options issued to him
         pursuant to the Option Agreement executed between himself and
         HighwayMaster Communications, Inc. dated as of June 13, 1994 (the
         "Option Agreement") will lapse and expire on June 30, 1999.

7.       In exchange for the promises in this Agreement, Mr. Quick on behalf of
         his heirs and assigns as well as himself, releases and discharges
         HighwayMaster from all of its duties, responsibilities and obligations
         of any kind under that certain Employment Agreement between Mr. Quick
         and HighwayMaster, dated as of November 23, 1994 (the "Employment
         Agreement"). HighwayMaster releases and discharges Mr. Quick from his
         duties, responsibilities and obligations of any kind under the
         Employment Agreement, except for the ongoing, prospective duties set
         forth in Section 5 of the Employment Agreement which are reconfirmed by
         this Agreement in return for the consideration set forth herein.

         HighwayMaster releases and discharges Mr. Quick from any and all
         claims, demands, losses, liabilities and causes of action arising or
         accruing concurrent with or prior to the date of execution of this
         Agreement, for or because of anything done or omitted by Mr. Quick.
         This release includes but is not limited to claims arising under Title
         VII of the Civil Rights Act of 1964, 42 U.S.C. Section 2000e (relating
         to employment discrimination), the Civil Rights Act of 1991, P.L.
         102-166, the Texas Human Relations Act, Tex. Rev. Viv. Stat. Art 5221k
         or similar statues from other applicable states, the Age Discrimination
         in Employment Act, 29 U.S.C. Section 621, and under any other state or
         federal statute or regulation, any claims for breach of contract, tort,
         and personal injury of any sort, including but not limited to those
         arising out of or relating in any way to Mr. Quick's employment by,
         association with and termination of employment from HighwayMaster. This
         release covers claims both that HighwayMaster knows about and those
         that it may not know about. Further, by accepting the benefits
         described above, HighwayMaster agrees not to sue Mr. Quick with respect
         to any claims that are released in this paragraph.

         Mr. Quick releases and discharges HighwayMaster from any and all
         claims, demands, losses, liabilities and causes of action arising or
         accruing concurrent with or prior to the date of execution of this
         Agreement, for or because of anything done or omitted by HighwayMaster.



                                      -2-
<PAGE>   3

         This release includes but is not limited to claims arising under Title
         VII of the Civil Rights Act of 1964, 42, U.S.C. Section 2000e (relating
         to employment discriminations) the Civil Rights Act of 1991, P.L.
         102-166, the Texas Human Relations Act, Tex. Rev. Civ. Stat. Art. 5221k
         or similar statutes from other applicable states, the Age
         Discrimination in Employment Act, 29 U.S.C. Section 621, and under any
         other state or federal statute or regulation, any claims for breach of
         contract, tort, and personal injury of any sort, including but not
         limited to those arising our of or relating in any way to Mr. Quick's
         employment by, association with and termination of employment from
         HighwayMaster. This release covers claims both that Mr. Quick knows
         about and those that he may not know about. Further, by accepting the
         benefits described above, Mr. Quick agrees not to sue HighwayMaster, or
         the related persons and entities described, with respect to any claims
         that are released in this paragraph

8.       Mr. Quick agrees that he will not at any time prior to May 1, 2000
         solicit for employment, hire under his direct or indirect managerial
         control, or encourage or refer any other party to solicit or hire for
         employment, any employee of HighwayMaster.

9.       By making this Agreement, HighwayMaster and Mr. Quick are not admitting
         that they have done anything wrong. HighwayMaster and Mr. Quick agree
         that this Agreement is inadmissible as evidence in any proceeding,
         legal or otherwise, except to the extent necessary to enforce its
         provisions.

         Mr. Quick will make no negative or disparaging statement, wither orally
         or in writing, about HighwayMaster or any of their directors, officers,
         shareholders, or affiliates. HighwayMaster and its management will make
         no negative or disparaging statement, either orally or in writing,
         about Mr. Quick.

10.      Mr. Quick acknowledges that he has been advised of his rights to
         consult his own attorney prior to signing this Agreement. Mr. Quick
         understands that whether or not to do so is Mr. Quick's decision. Mr.
         Quick agrees, however, that HighwayMaster shall not be required to pay
         any of his attorney's fees or costs, now or later, in this or any
         related matter or lawsuit, except to the extent of $1,500 in connection
         with the preparation and negotiation of this Agreement which shall be
         payable with five (5) days of receipt of any attorney's invoice
         therefor, and provided, further, that any attorney's fees to which Mr.
         Quick is otherwise legally entitled for the enforcement of this
         Agreement or for which the Company is legally required to pay under its
         Bylaws or insurance policies are not waived by this Section 10 of this
         Agreement.

11.      This Agreement constitutes the entire agreement and understanding
         between the parties with respect to the subject matter herein, and
         supersedes and replaces any and all prior agreements and
         understandings, whether oral or written, with respect to such matters.
         Any representation or statement not expressly contained in this
         Agreement will not be binding upon either party. No amendment,
         interpretation or waiver of any of the provisions of this Agreement
         shall be effective unless made in writing and signed by the parties
         hereto. If any portion of this Agreement is found to be unenforceable,
         then both Mr. Quick and HighwayMaster desire that all other portions
         that can be separated from it or appropriately limited in scope will
         remain fully valid and enforceable. Each party also agrees that,
         without 



                                      -3-
<PAGE>   4

         receiving further consideration, it will sign and deliver such
         documents and do anything else that is reasonably necessary in the
         future to make the provisions of this Agreement effective.

12.      This Agreement is to be executed, delivered, and performed in Dallas,
         Texas. This Agreement shall be construed in accordance with the laws of
         the State of Texas. In the event any dispute or controversy arises
         regarding the terms, enforceability, subject matter, or any other
         controversy arises out of or is related to this Agreement, the parties
         agree that such dispute or controversy shall be submitted exclusively
         to binding arbitration pursuant to the rules and regulations of the
         American Arbitration Association (the "AAA"). The parties agree that
         should arbitration become necessary, they will utilize and comply with
         all available rules of the AAA for expediting such arbitration. The
         site of such arbitration will be in Dallas County, Texas and will
         commence no later than 30 days after any party files for arbitration.

13.      MR. QUICK ACKNOWLEDGES THAT HE HAS READ THIS AGREEMENT, HAS HAD AN
         OPPORTUNITY TO ASK QUESTIONS AND HAVE IT EXPLAINED TO HIM AND THAT HE
         UNDERSTANDS THAT THE AGREEMENT WILL HAVE THE EFFECT OF WAIVING ANY
         ACTION HE MIGHT PURSUE, INCLUDING BREACH OF CONTRACT, PERSONAL INJURY,
         DISCRIMINATION ON THE BASIS OF RACE, AGE, SEX NATIONAL ORIGIN, OR
         DISABILITY AND ANY OTHER CLAIMS ARISING PRIOR TO THE EFFECTIVE DATE OF
         THE AGREEMENT. Mr. Quick represents that he has not been induced to
         execute this Agreement by any statement, act or representation of any
         kind or character on the part of anyone, except as may be contained in
         this Agreement.

         ACCEPTED AND AGREED TO:


         /s/ GORDON QUICK                               DATE:    12/21/98   
         -----------------------------                       ----------------
         GORDON QUICK


         HIGHWAYMASTER CORPORATION


         BY:  /s/ JANA BELL                             DATE:    12/22/98
            --------------------------                       ----------------
         NAME:    Jana Bell                                   
              ------------------------  
         TITLE:   President and CEO                      
               -----------------------  


         HIGHWAYMASTER COMMUNICATIONS, INC.

         BY:  /s/ JANA BELL                             DATE:    12/22/98
            --------------------------                       ----------------
         NAME:    Jana Bell                                   
              ------------------------  
         TITLE:   President and CEO                      
               -----------------------  



                                      -4-