<Page> AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON NOVEMBER 21, 2002 REGISTRATION NO. 333- - -------------------------------------------------------------------------------- - -------------------------------------------------------------------------------- SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 ------------------------------ FORM S-1 REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933 ------------------------------ SAFETY INSURANCE GROUP, INC. (Exact name of Registrant as specified in its charter) <Table> DELAWARE 6331 13-4181699 (State or other jurisdiction (Primary Standard (I.R.S. Employer of Industrial Classification Code Number) Identification No.) incorporation or organization) </Table> 20 CUSTOM HOUSE STREET BOSTON, MA 02110 (617) 951-0600 (Address, including zip code, and telephone number, including area code, of Registrant's principal executive offices) ------------------------------ WILLIAM J. BEGLEY, JR. CHIEF FINANCIAL OFFICER AND SECRETARY SAFETY INSURANCE GROUP, INC. 20 CUSTOM HOUSE STREET BOSTON, MA 02110 (617) 951-0600 (Name, address, including zip code, and telephone number, including area code, of agent for service) ------------------------------ COPIES TO: <Table> Robert S. Rachofsky Jeff S. Liebmann LeBoeuf, Lamb, Greene & MacRae, L.L.P. Jonathan L. Freedman 125 West 55th Street Dewey Ballantine LLP New York, NY 10019-5389 1301 Avenue of the Americas New York, NY 10019-6092 </Table> ------------------------------ Approximate date of commencement of the proposed sale of the securities to the public: As soon as practicable after the Registration Statement becomes effective. If any of the securities being registered on this form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, check the following box. / / If this form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. /X/ 333-87056 If this form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. / / If this form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. / / If delivery of the prospectus is expected to be made pursuant to Rule 434, check the following box. / / ------------------------------ CALCULATION OF REGISTRATION FEE <Table> <Caption> PROPOSED MAXIMUM PROPOSED MAXIMUM TITLE OF EACH CLASS OF SECURITIES TO BE AMOUNT TO BE OFFERING PRICE PER AGGREGATE OFFERING AMOUNT OF REGISTERED REGISTERED(1) SHARE(2) PRICE(2) REGISTRATION FEE(3) Common Stock, $.01 par value 7,400,000 $12.00 $88,800,000 $11,556 </Table> (1) Includes 900,000 shares of Common Stock that may be sold pursuant to the Underwriters' over-allotment option. (2) This Registration Statement relates to the Registrant's Registration Statement on Form S-1 (Registration No. 333-87056) (the "Prior Registration Statement"). In accordance with Rule 462(b) under the Securities Act of 1933, as amended, the amount of securities registered under the Prior Registration Statement (6,900,000 shares of common stock) is carried forward to this Registration Statement, and an additional amount of securities (500,000 shares of common stock) is registered hereby. (3) This amount includes $11,004 which the registrant has previously paid toward the registration fee for the prior Registration Statement. ------------------------------ - -------------------------------------------------------------------------------- - -------------------------------------------------------------------------------- <Page> Incorporation By Reference of Registration Statement on Form S-1, File No. 333-87056 Pursuant to Rule 462(b) under the Securities Act of 1933, as amended, Safety Insurance Group, Inc. (the "Registrant") hereby incorporates by reference into this Registration Statement on Form S-1 in its entirety the Registration Statement on Form S-1 (File No. 333-87056) declared effective on November 12, 2002 by the Securities and Exchange Commission (the "Commission"), including the exhibits and power of attorney thereto and each of the documents filed by the Registrant with the Commission and incorporated or deemed to be incorporated by reference therein. II-1 <Page> EXHIBITS The following exhibits are filed herewith: <Table> <Caption> EXHIBIT NUMBER DESCRIPTION - --------------------- ----------- 5 Opinion of LeBoeuf, Lamb, Greene & MacRae, L.L.P. regarding the legality of the securities being registered (incorporated by reference to Exhibit 5 to the Registration Statement on Form S-1 (File No. 333-87056)) 23.1 Consent of PricewaterhouseCoopers LLP 23.2 Consent of LeBoeuf, Lamb, Greene & MacRae, L.L.P. (incorporated by reference to Exhibit 5 to the Registration Statement on Form S-1 (File No. 333-87056)) 24 Power of Attorney (incorporated by reference to Exhibit 24 to the Registration Statement on Form S-1 (File No. 333-87056)) </Table> II-2 <Page> SIGNATURES Pursuant to the requirements of the Securities Act of 1933, the registrant has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the city of Boston, Commonwealth of Massachusetts, on November 21, 2002. <Table> SAFETY INSURANCE GROUP, INC. By: /s/ WILLIAM J. BEGLEY, JR. ---------------------------------------- Name: William J. Begley, Jr. Title: Chief Financial Officer, Vice President and Secretary </Table> Pursuant to the requirements of the Securities Act of 1933, this Registration Statement has been signed below by the following persons in the capacities and on the dates indicated. <Table> <Caption> SIGNATURE TITLE DATE --------- ----- ---- * Chief Executive Officer, ------------------------------------------- President November 21, 2002 David F. Brussard and Director /s/ WILLIAM J. BEGLEY, JR. Chief Financial Officer, ------------------------------------------- Vice President and November 21, 2002 William J. Begley, Jr. Secretary * ------------------------------------------- Director November 21, 2002 A. Richard Caputo, Jr. * ------------------------------------------- Director November 21, 2002 John W. Jordan II * ------------------------------------------- Director November 21, 2002 David W. Zalaznick </Table> <Table> *By: /s/ WILLIAM J. BEGLEY, JR. -------------------------------------- November 21, 2002 AS ATTORNEY-IN-FACT </Table> II-3