From "The United States Access Board" website.......
These standards, as adopted by the Department of Justice (DOJ) in September 2010, will take effect March 15, 2012 and replace DOJ’s original ADA standards. DOJ is allowing immediate use of the 2010 standards as an alternative to the original 1991 standards. DOJ’s standards apply to facilities covered by the ADA, including places of public accommodation, commercial facilities, and state and local government facilities. They do not apply to transportation facilities, including bus stops and rail stations, which are subject to similar standards issued by the Department of Transportation.
703.2 Raised Characters. Raised characters shall comply with 703.2 and shall be duplicated in braille complying with 703.3. Raised characters shall be installed in accordance with 703.4.
Advisory 703.2 Raised Characters. Signs that are designed to be read by touch should not have sharp or abrasive edges.
703.2.1 Depth. Raised characters shall be 1/32 inch (0.8 mm) minimum above their background.
703.2.2 Case. Characters shall be uppercase.
703.2.3 Style. Characters shall be sans serif. Characters shall not be italic, oblique, script, highly decorative, or of other unusual forms.
703.2.4 Character Proportions. Characters shall be selected from fonts where the width of the uppercase letter "O" is 55 percent minimum and 110 percent maximum of the height of the uppercase letter "I".
703.2.5 Character Height. Character height measured vertically from the baseline of the character shall be 5/8 inch (16 mm) minimum and 2 inches (51 mm) maximum based on the height of the uppercase letter "I".
EXCEPTION: Where separate raised and visual characters with the same information are provided, raised character height shall be permitted to be 1/2 inch (13 mm) minimum.
Figure 703.2.5 Height of Raised Characters 703.2.6 Stroke Thickness. Stroke thickness of the uppercase letter "I" shall be 15 percent maximum of the height of the character.
703.2.7 Character Spacing. Character spacing shall be measured between the two closest points of adjacent raised characters within a message, excluding word spaces. Where characters have rectangular cross sections, spacing between individual raised characters shall be 1/8 inch (3.2 mm) minimum and 4 times the raised character stroke width maximum. Where characters have other cross sections, spacing between individual raised characters shall be 1/16 inch (1.6 mm) minimum and 4 times the raised character stroke width maximum at the base of the cross sections, and 1/8 inch (3.2 mm) minimum and 4 times the raised character stroke width maximum at the top of the cross sections. Characters shall be separated from raised borders and decorative elements 3/8 inch (9.5 mm) minimum.
703.2.8 Line Spacing. Spacing between the baselines of separate lines of raised characters within a message shall be 135 percent minimum and 170 percent maximum of the raised character height.