Document:

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                               XTRA CORPORATION

                         GENERAL STOCK INCENTIVE PLAN

(1)  Purpose

       The purpose of this General Stock Incentive Plan (the "Plan") is to
advance the interests of XTRA Corporation (the "Company") by enhancing the
ability of the Company (a) to attract and retain employees who are in a position
to make significant contributions to the success of the Company; (b) to reward
employees for such contributions; and (c) to encourage employees to take into
account the long-term interests of the Company through ownership of shares of,
and other interests in, the Company's common stock ("Common Stock").

       The Plan is intended to accomplish these goals by enabling the Company to
grant awards ("Awards") to eligible employees. Awards may be in the form of
Stock Options (as described in Section 6), Stock Appreciation Rights (as
described in Section 7) and Restricted Stock Awards (as described in Section 8).

(2)  Administration

       The Plan will be administered by the Compensation Committee of the Board
of Directors of the Company (the "Committee"). The Committee will have
authority, not inconsistent with the express provisions of the Plan, (a) to
grant Awards to such eligible employees as the Committee may select
("Participants"); (b) to determine the type of Awards to be granted and the
times of grants; (c) to determine the number of shares of Common Stock to be
covered by any Award; (d) to determine the terms and conditions of any Award,
which terms and conditions may differ among individual Awards and Participants;
(e) to prescribe the form or forms of instruments evidencing Awards and any
other instruments required under the Plan and to change such forms from time to
time; (f) to adopt, amend and rescind rules and regulations for the
administration of the Plan; (g) to interpret the Plan and to decide any
questions and settle all controversies and disputes that may arise in connection
with the Plan; and (h) to waive compliance by a Participant with any obligation
to be performed by him under an Award. Such determinations and actions of the
Committee shall be conclusive and shall bind all parties.

       A majority of the members of the Committee will constitute a quorum, and
all determinations of the Committee shall be made by a majority of its members.
Any determination of the Committee under the Plan may be made without notice or
meeting of the Committee by a writing signed by a majority of the Committee
members. All members of the Committee shall be non-employee directors within the
meaning of Rule 16b-3 under the Securities Exchange Act of 1934.
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(3)  Effective Date

       The effective date of the Plan is December 1, 1998.

(4)  Shares Subject to the Plan

     (a)  Number of Shares. Subject to adjustment as provided in Section 10, the
aggregate number of shares of Common Stock that may be delivered under the Plan
is 150,000. Shares of Common Stock may be issued up to this maximum pursuant to
any type or types of Award. For purposes of this limitation, Awards and shares
of Common Stock which are forfeited or reacquired by the Company, and Awards
which are satisfied without the issuance of shares of Common Stock, will not be
counted. Such limitation will apply only to shares of Common Stock which have
become free of any restrictions under the Plan.

               (b)  Shares to be Delivered. Shares delivered under the Plan will
          be authorized but unissued shares of Common Stock or, if the Committee
          so decides in its sole discretion, previously issued Common Stock
          acquired by the Company and held in treasury. No fractional shares of
          Common Stock will be delivered under the Plan.

(5)  Eligibility

       Employees eligible to become Participants shall be those key employees of
the Company and its subsidiaries who, in the opinion of the Committee, are in a
position to make a significant contribution to the success of the Company or its
subsidiaries. A subsidiary for purposes of the Plan is a corporation in which
the Company owns, directly or indirectly, stock possessing 50% or more of the
total combined voting power of all classes of stock. Members of the Committee
will not be eligible to become Participants.

(6)  Stock Options

       Stock Options granted under the Plan ("Options") shall be non-qualified
stock options ("NSOs") -- that is, options that are not intended to qualify
under Section 422 of the Internal Revenue Code.

       Options granted under the Plan will be subject to the following terms and
conditions and will contain such additional terms and conditions, not
inconsistent with the terms of the Plan, as the Committee deems desirable:

               (a)  Exercise Price. The exercise price of each Option will be
          determined by the Committee but may not be less than 100% of the fair
          market value per share of Common Stock at the time the Option is
          granted. For purposes of the preceding sentence, "fair market value"
          on any given date means the highest closing sale price on the date
          immediately preceding the date in question as reflected in the New
          York Stock

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Exchange Composite Index. For purposes of this subsection, an option will be
treated as having been granted as of the date specified as the date of grant by
the Committee.

     (b)  Duration of Options. An Option will be exercisable during such period
or periods as the Committee may specify. The latest date on which an Option may
be exercised will be the date which is ten (10) years from the date the Option
was granted or such earlier date as may be specified by the Committee at the
time the Option is granted.

     (c)  Exercise of Options.

     (i)  Options will be exercisable at such future time or times, whether or
not in installments, as determined by the Committee at or after the grant date.
The Committee may at any time accelerate the exercisability of all or any
portion of any Option.

     (ii) Any exercise of an Option must be by written notice to the Company,
accompanied by (i) the document evidencing the Option (the "Option Certificate")
and any other documents required by the Committee and (ii) payment in accordance
with Section 6(d) below for the number of shares of Common Stock for which the
Option is exercised.

     (d)  Payment for and Delivery of Common Stock. Common Stock purchased on
exercise of an Option shall be paid for as follows: (1) in cash or by certified
check, bank draft or money order payable to the order of the Company or (2) if
so permitted by the Option Certificate, (i) through the delivery of shares of
Common Stock (held for at least six months, or such other period as the
Committee may specify) having a fair market value on the last business day
preceding the date of exercise equal to the purchase price or (ii) by a
combination of cash and Common Stock as provided in clauses (1) and (2)(i) above
or (iii) by delivery of a promissory note of the Participant to the Company,
payable on such terms as are specified in the Option Certificate (except that
the Option Certificate may provide that the rate of interest on the note will be
the lowest rate which is sufficient, at the time the note is given, to avoid
imputation of interest under the applicable provisions of the Code), or by a
combination of cash (or cash and Common Stock) and the Participant's promissory
note; provided, that if the Common Stock delivered upon exercise of the Option
is an original issue of authorized Common Stock, at least so much of the
exercise price as represents the par value of such Common Stock must be paid in
cash if the Committee determines that cash payment is required by law.

     (e)  Nontransferability of Options. Except as may otherwise be determined
by the Committee, no Option may be transferred other than by will or by the laws
of

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     descent and distribution, and during a Participant's lifetime an Option may
     be exercised only by the Participant.

          (f)  Death or Disability. If a Participant's employment with the
     Company and its subsidiaries terminates by reason of death or total and
     permanent disability, each Option held by the Participant will become fully
     exercisable and will remain exercisable after the date of such termination
     for a period of two years in the case of death and one year in the case of
     disability (but in no event later than the date the option would have
     expired in all events under Section 6(b)). In the case of a deceased
     Participant, such Option may be exercised within such time limits by the
     executor or administrator of the deceased Participant's estate, or by the
     person or persons to whom the Option is transferred by will or the
     applicable laws of descent and distribution.

          (g)  Other Termination of Employment. If a Participant's employment
     with the Company and its subsidiaries terminates for any reason other than
     death or total and permanent disability, all Options held by the
     Participant that are not then exercisable shall terminate. Options that are
     exercisable on the date of termination will continue to be exercisable for
     a period of three months (but in no event later than the date the option
     would have expired in all events under Section 6(b)) unless the employee
     has confessed to, or been convicted of, any act of fraud, theft or
     dishonesty arising in the course of, or in connection with, his employment
     with the Company, in which case the Option will terminate immediately and
     in full. After completion of that three-month (or shorter) period such
     Options shall terminate to the extent not previously exercised, expired or
     terminated.

(7)  Stock Appreciation Rights

          (a)  Nature of Stock Appreciation Right. A Stock Appreciation Right
     ("SAR") is an Award entitling the recipient to receive an amount in cash or
     shares of Common Stock or a combination thereof having a value equal to the
     excess of the fair market value of a share of Common Stock on the date of
     exercise over the fair market value of a share of Common Stock on the date
     of grant (or over the Option exercise price, if the SAR was granted in
     tandem with an Option) multiplied by the number of shares with respect to
     which the SAR has been exercised, with the Committee having the right to
     determine the form of payment.

          (b)  Grant of SARs. SARs may be granted in tandem with, or
     independently of, Options granted under the Plan. An SAR granted in tandem
     with an Option may be granted either at or after the time of the grant of
     such Option. SARs will be evidenced by such written agreement as is deemed
     appropriate by the Committee.

     An SAR or applicable portion thereof granted in tandem with an Option will
terminate and no longer be exercisable upon the termination or exercise of such
Option, except that an

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SAR granted with respect to less than the full number of shares covered by an
Option will not be reduced until the exercise or termination of the related
Option exceeds the number of shares not covered by the SAR.

          (c)    Terms and Conditions of SARs. SARs will be subject to such
     terms and conditions as are determined from time to time by the Committee,
     subject, in the case of SARs granted in tandem with Options, to the
     following:

          (i)    SARs will be exercisable only at such time or times and to the
     extent that the related Option is exercisable.

          (ii)   Upon the exercise of an SAR, the applicable portion of any
     related Option must be surrendered.

          (iii)  SARs will be transferable only with the related Option. All
     SARs will be exercisable during the Participant's lifetime only by the
     Participant or his legal representative.

          (iv)   An SAR granted in tandem with an Option may be exercised only
     when the market price of the Common Stock subject to the Option exceeds the
     exercise price of such Option.

     The provisions of Sections 6(f) and 6(g) relating to the exercisability and
termination of Options shall also apply to SARs, whether or not granted in
tandem with Options.

     Any exercise of an SAR must be by written notice to the Company,
accompanied by the document evidencing the SAR and any other documents required
by the Committee.

(8)  Restricted Stock

          (a)    Nature of Restricted Stock Award. A Restricted Stock Award is
     an Award entitling the recipient to acquire shares of Common Stock
     ("Restricted Stock") for a purchase price (which may be zero) not to exceed
     par value, subject to such conditions, including the restrictions specified
     in Section 8(d) below, as the Committee may impose at the time of grant.

          (b)    Award Agreement. A Participant who is granted a Restricted
     Stock Award will have no rights with respect to such award unless the
     Participant accepts the Award within 60 days (or such other period as the
     Committee may specify) following the award date by making payment to the
     Company by certified or bank check or other instrument acceptable to the
     Committee in an amount equal to the specified purchase price, if any, of
     the shares covered by the Award and by executing and delivering to the

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     Company an agreement (an "Award Agreement") in such form as the Committee
     determines.

          (c)    Rights as a Stockholder. Upon complying with Section 8(b)
     above, a Participant will have all the rights of a stockholder with respect
     to the Restricted Stock awarded to him including voting and dividend
     rights, subject to the restrictions described in this section 8 and subject
     to any other conditions contained in the award Agreement. Unless the
     Committee otherwise determines, certificates evidencing shares of
     Restricted Stock will remain in the possession of the Company until such
     shares are free of any restrictions under the Plan.

          (d)    Restriction. Shares of Restricted Stock may not be sold,
     assigned, transferred, pledged or otherwise encumbered or disposed of
     except as specifically provided herein. If a Participant ceases for any
     reason to be employed by the Company or its subsidiaries, shares of
     Restricted Stock held by such Participant shall be resold to the Company at
     their purchase price, or forfeited to the Company if the purchase price was
     zero, except as specifically set forth herein. Shares of Restricted Stock
     resold to the Company shall have the status of authorized but unissued
     shares of Common Stock.

          (i)    The Committee will specify in the Award Agreement the date or
     dates (which may depend upon or be related to the attainment of performance
     goals or other conditions) on which the nontransferability of the
     Restricted Stock and the obligation of the Participant to resell such Stock
     to the Company will lapse. The Committee may at any time accelerate such
     date or dates.

          (ii)   If the Participant's employment terminates because of death or
     total and permanent disability, all restrictions on Restricted Stock held
     by the Participant will lapse.

          (e)    Notice of Election. Any Participant making an election under
     section 83(b) of the Code with respect to a Restricted Stock Award must
     provide a copy thereof to the Company within 30 days of the filing of such
     election with the Internal Revenue Service.

(9)  Cash Awards

     In connection with any Award hereunder the Committee may, in its sole
discretion, at the time such Award is made or at a later date, provide for and
grant a cash award to the Participant not to exceed an amount equal to (a) the
amount of any federal, state and local income tax on ordinary income for which
the Participant will be liable with respect to the Award, plus (b) an additional
amount on a grossed-up basis necessary to make him whole after tax, discharging
all his income tax liabilities arising form all payments under this Section 9.

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Any payments under this Section 9 will be made at the time the Participant
incurs federal income tax liability with respect to the Award.

(10) Changes in Company; Substitute Awards

          (a)    Changes in Stock. In the event of a stock dividend, stock split
     or combination of shares, recapitalization or other change in the Company's
     capital stock, the number and kind of shares of stock or securities of the
     Company subject to awards then outstanding or subsequently granted under
     the Plan, the maximum number of shares of stock or securities that may be
     delivered under the Plan, the purchase price, and other relevant provisions
     will be appropriately adjusted by the Committee, whose determination shall
     be binding on all persons.

     The Committee may also adjust the number of shares subject to outstanding
Awards, the exercise price of outstanding Options and the terms of outstanding
Awards, to take into consideration material changes in accounting practices or
principles, consolidations or mergers (except those described in Section 10(b)
below), acquisitions or dispositions of stock or property or any other event if
it is determined by the Committee that such adjustment is appropriate to avoid
distortion in the operation of the Plan.

          (b)    Merger, Etc. In the event of a dissolution or liquidation of
     the Company or a merger or consolidation in which the Company is not the
     surviving corporation or its outstanding shares are converted into
     securities of another corporation or exchanged for other consideration, all
     Options and SARs granted hereunder will terminate, but at least 20 days
     prior to the effective date of any such dissolution or liquidation (or 20
     days prior to any earlier related sale of substantially all the assets of
     the Company) or of such merger or consolidation, the Committee may arrange
     that the successor or surviving corporation, if any, grant replacement or
     substitute replacement Options and/or SARs.

          (c)    Substitute Awards. The Company may grant Awards under the Plan
     in substitution for stock and stock based awards held by employees of
     another corporation who concurrently become employees of the Company or a
     subsidiary as the result of a merger or consolidation of the employing
     corporation with the Company or a subsidiary or the acquisition by the
     Company or a subsidiary of property or stock of the employing corporation.
     The Committee may direct that the substitute Awards be granted on such
     terms and conditions as the Committee considers appropriate.

(11) General Provisions

          (a)    No Distribution; Compliance with Legal Requirements, Etc. The
     Committee may require each person acquiring Common Stock pursuant to an
     Award to

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     represent to and agree with the Company in writing that such person is
     acquiring the Common Stock without a view to distribution thereof.

     The Company will not be obligated to deliver any shares of Common Stock
pursuant to an Award (1) until, in the opinion of the Company's counsel, all
applicable federal and state laws and regulations have been complied with, and
(2) if the outstanding Common Stock is at the time listed on any stock exchange,
until the shares to be delivered have been listed or authorized to be listed on
such exchange upon official notice of issuance, and (3) until all other legal
matters in connection with the issuance and delivery of such shares have been
approved by the Company's counsel. If the sale of Common Stock has not been
registered under the Securities Act of 1933, as amended, the Company may require
such representations or agreements as counsel for the Company may consider
appropriate to avoid violation of such Act and may require that the certificates
evidencing such Common Stock bear an appropriate legend restricting transfer.

     Notwithstanding any provision of the Plan, the Company will be under no
obligation to deliver shares of Common Stock to an estate of a deceased
Participant, or to the person or persons to whom the Award has been transferred
by the Participant's will or the applicable laws of descent and distribution,
until the Company is satisfied as to the authority of such person or persons.

          (b)    Tax Withholding, Etc. Each Participant will, no later than the
     date as of which the value of an Award or of any Common Stock or other
     amounts received hereunder first becomes includable in gross income for
     federal income tax purposes, pay to the Company, or make arrangements
     satisfactory to the Committee regarding payment of, all federal, state and
     local taxes required by law to be withheld with respect to such income. The
     Company and its subsidiaries will, to the extent permitted by law, have the
     right to deduct any such taxes from any payment of any kind otherwise due
     to the Participant.

     The Committee may provide, in respect of any transfer of Common Stock under
an Award, that if and to the extent withholding of any federal, state or local
tax is required, the Participant may elect in such manner as the Committee
prescribes, to have the Company hold back from the transfer Common Stock having
a value calculated to satisfy such withholding obligation, or to deliver to the
Company previously owned shares of equal value. Notwithstanding the foregoing,
in the case of a Participant subject to the restrictions of Section 16(b) of the
Securities Exchange Act of 1934 no such election shall be effective unless made
in compliance with any applicable requirements of Rule 16b-3(e) or any successor
rule under such Act.

          (c)    Continuance of Employment. For purposes of the Plan, employment
     of a Participant will not be considered terminated (1) in the case of sick
     leave or other bona fide leave of absence approved for purposes of the Plan
     by the Committee, so long

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     as the Participant's right to reemployment is guaranteed either by statute
     or by contract, or (2) in the case of a transfer to the employment of a
     corporation (or a parent or subsidiary corporation of such corporation)
     issuing or assuming an option in a transaction to which section 424(a) of
     the Code would apply.

          (d)    Fair Market Value. Except as set forth herein, for purposes of
     the Plan, in general, "fair market value" of a share of Common Stock on any
     date means the closing price on such date as reflected in the New York
     Stock Exchange Composite Index. If, however, the Committee determines that
     a different meaning is in any circumstance necessary in order to comply
     with applicable law, such different meaning will apply in that
     circumstance.

          (e)    Employment Rights. Neither the adoption of the Plan nor the
     grant of Awards will confer upon any employee any right to continued
     employment with the Company or any subsidiary or affect in any way the
     right of the Company or subsidiary to terminate the employment of an
     employee at any time. Except as specifically provided by the Committee in
     any particular case, the loss of existing or potential profit in Awards
     granted under this Plan shall not constitute an element of damages in the
     event of termination of the employment of an employee even if the
     termination is in violation of an obligation of the Company to the employee
     by contract or otherwise.

(12) Effect, Discontinuance, Cancellation, Amendment and Termination.

     Neither adoption of the Plan nor the grant of awards to a Participant shall
affect the Company's right to grant to such Participant awards that are not
subject to the Plan, to issue to such Participant Common Stock as a bonus or
otherwise, or to adopt other plans or arrangements under which Common Stock may
be issued to employees.

     The Committee may at any time discontinue granting Awards under the Plan.
With the consent of the Participant, the Committee may at any time cancel an
existing Award in whole or in part and grant the Participant another Award for
such number of shares of Common Stock as the Committee specifies, subject to
Section 4(b). The Committee may at any time or times amend the Plan or any
outstanding Award for the purpose of satisfying the requirements of any changes
in applicable laws or regulations or for any other purpose which may at the time
be permitted by law or may at any time terminate the Plan as to any further
grants of Awards.

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                                                                    EXHIBIT 4.12

                                JOINDER AGREEMENT
                            TO THE SECOND AMENDED AND
                     RESTATED REGISTRATION RIGHTS AGREEMENT
                          OF SPECTRASITE HOLDINGS, INC.

                                            December 14, 2000

c/o SBC Communications Inc.
SBC Tower Holdings, LLC
175 E. Houston
San Antonio, TX  78205
Attention:  Senior Executive Vice President - Corporate Development

Ladies and Gentlemen:

              Reference is made to that certain Second Amended and Restated
Registration Rights Agreement dated April 20, 1999 among SpectraSite Holdings,
Inc. (the "COMPANY"), the WCAS Purchasers, the Whitney Purchasers, the CIBC
Purchasers, the Additional Purchasers, including TPC (each as defined therein),
as amended by the Joinder Agreement to the SpectraSite Restated Registration
Rights Agreement, dated January 5, 2000, among the Company and the Apex
Stockholders (as defined therein), as further amended by the Consent and
Agreement to SBCW Registration Rights and Amendment to the Existing Registration
Rights Agreement dated November 20, 2000 (such agreements, collectively, being
referred to herein as the "EXISTING REGISTRATION RIGHTS AGREEMENT" and the
Purchasers thereunder being referred to as the "EXISTING HOLDERS"). Reference is
also made to that certain Agreement to Sublease, dated as of August 25, 2000
(the "SBC AGREEMENT") among the Company, Southern Towers, Inc., SBC Wireless,
Inc., for itself and on behalf of the Sublessor Entities, ("SBCW") pursuant to
which SBC Tower Holdings LLC ("SBCT") will be entitled to become a stockholder
of the Company. The Company and SBCT agree as follows:

              1.     By SBCT's execution of this Joinder Agreement, subject to
the terms and conditions set forth herein, (a) SBCT will become a party to the
Existing Registration Rights Agreement as a holder of "SBCT Restricted Stock"
(as defined below), (b) the Parent Stock (as defined in the Agreement) (the
"SBCT RESTRICTED STOCK") shall be treated as "Restricted Stock" as that term is
used in the Existing Registration Rights Agreement for all purposes thereof, and
(c) SBCT will be entitled to all of the benefits of and subject to all of the
obligations of a holder of "Restricted Stock" under the Existing Registration
Rights Agreement as if it had been an original party thereto.

<PAGE>   2

              2.     Section 5 is hereby amended by deleting the first sentence
of Section 5(a) and replacing it in its entirety with the following:

              "If, at any time when Form S-3 is available for such registration
and subject to the terms of Section 2 of the Stockholders' Agreement, the
Company shall receive from any Institutional Investor, SBCT or TPC a written
request that the Company effect a registration on Form S-3 of any such holder's
Restricted Stock, the Company will promptly give written notice of the proposed
registration to all other holders of Restricted Stock, and , as soon as
practicable, effect such registration and all such related qualifications and
compliances as may be reasonably requested and as would permit or facilitate the
sale and distribution of all Restricted Stock as are specified in such request
and any written requests of other holders given within 20 days after receipt of
such notice."

              3.     For purposes of Sections 4, 5 and 6 of the Existing
Registration Rights Agreement, it is understood that with respect to any sales
of Restricted Stock in an underwritten registered public offering under such
sections (other than a registration initiated by SBCT) SBCT's obligation to
include in such registration its Restricted Stock pursuant to a request to
include such stock in such offering in accordance with the provisions of Section
4, 5 or 6, may be conditioned, at SBCT's option (by expressly stating so in its
request), upon such offering qualifying as a Liquidity Event (as defined in the
SBC Agreement).

              4.     The Company intends to file a registration statement on
Form S-3 (or other appropriate form available for registering stock for sale to
the public) to register the resale by SBCT of $25,000,000 of Restricted Stock as
contemplated by Section 5.14 of the SBC Agreement.

              5.     The Company represents and warrants to the undersigned as
follows:

                     (a)    The Company has full corporate power and authority
to execute and deliver this Joinder Agreement and to perform its obligations
hereunder. This Joinder Agreement constitutes the valid and legally binding
obligation of the Company, enforceable in accordance with its terms and
conditions, subject to bankruptcy, insolvency, reorganization, moratorium or
similar laws now or hereafter in effect affecting creditors' rights generally.
The Company does not need to give any notice to, make any filing with, or obtain
any authorization, consent, or approval of any other party in order to
consummate the transactions contemplated by this Joinder Agreement other than
such notices, filings, authorizations, consents and approvals as have been
obtained prior to the date hereof.

                     (b)    Neither the execution and the delivery of this
Joinder Agreement, nor the consummation of the transactions contemplated hereby,
(i) violate any injunction, judgment, order, decree, ruling, charge, or other
restriction of any governmental authority to which the Company is subject or any
provision of its charter or bylaws or (ii) other than such notices, filings,
authorizations, consents and approvals as have been obtained and after giving
effect to them, violate, result in a breach of, constitute a default under,
result in the acceleration of, create in any party the right to accelerate,
terminate, modify, or cancel, or require any notice under any agreement,
contract, lease, license, instrument, or other arrangement to which the Company
is a party or by which it is bound or to which any of its assets is subject.

                                       2
<PAGE>   3

              6.     Notices, requests, consents and other communications mailed
pursuant to Section 13(c) of the Existing Registration Rights Agreement, if
mailed to SBCT, shall be mailed to:

                     SBC Tower Holdings LLC
                     c/o SBC Communications Inc.
                     175 E. Houston, 12th Floor
                     San Antonio, TX  78205
                     Attention:  Senior Executive Vice President -
                            Corporate Development

                     with a copy to:

                     SBC Communications Inc.
                     175 E. Houston, 4th Floor
                     San Antonio, TX  78205
                     Attention:  Vice President and Assistant General Counsel-
                            Mergers and Acquisitions

              7.     This Joinder Agreement shall be governed by and construed
in accordance with the laws of the State of New York without regard to
principles of conflicts of law of such state.

              8.     This Joinder Agreement, together with the Existing
Registration Rights Agreement as modified hereby, constitutes the entire
agreement of the parties with respect to the subject matter hereof. Hereafter,
the Joinder Agreement shall be deemed part of and incorporated into the Existing
Registration Rights Agreement, and the Existing Registration Rights Agreement
(including this Joinder Agreement) may be modified or amended only in accordance
with Section 13(e) of the Existing Registration Rights Agreement, taking into
account the SBCW Restricted Stock then outstanding in measuring the 60%
threshold provided therein, except that any modification or amendment that
adversely affects the rights or privileges of SBCT and does not affect the other
holders of Restricted Stock and Management Stock in a substantially similar
manner shall require the consent of SBCT.

              9.     This Joinder Agreement may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

                   REMAINDER OF PAGE INTENTIONALLY LEFT BLANK

                                       3
<PAGE>   4

              Please indicate the acceptance of the foregoing by signing and
returning the enclosed counterpart of this letter, whereupon this letter (herein
sometimes called the "Joinder Agreement") shall be a binding agreement between
the parties set forth below.

                                            Very truly yours,

                                            SPECTRASITE HOLDINGS, INC.

                                            By:    /s/ Richard J. Byrne
                                                --------------------------------
                                                Richard J. Byrne
                                                Executive Vice President

AGREED TO AND ACCEPTED,
this 14 day of December, 2000

SBC TOWER HOLDINGS LLC

By:     New Southwestern Bell Mobile Systems, Inc.,
        its Managing Member

By:    /s/ Gregory L. Gibson
    ----------------------------------------
    Name: Gregory L. Gibson
    Title: Attorney-in-fact

                                       4

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