Negative grade makes the stroke thinner, while positive grade makes it thicker. In the case of Highway VAR.2 the static font starts at the original stroke thickness (0), and has two grades named G-1 (-100) and G-2 (-200). The variable font can be adjusted seamlessly between 0 and -200 to achieve a tailored adjustment for reversed out type, and to make type more legible at small sizes. More information is available in the Specimen PDF included in the download file.
Issues with halation (further heightened by the glare of headlights at night) were one of the reasons the Federal Highway Administration justified a transition from the Standard Alphabets to a new typeface for highway signs known as “Clearview”. Although it was easier to read from a distance the new font met resistance and was soon abandoned. Maybe they just needed a grade or two. Read more here.
The Standard Alphabets for Highway Signs were designed by the U.S. Public Roads Administration (which later became FHWA) and first adopted in the USA between the late 1940s and early 1950s. They replaced the original nationally standardized “Joint Board” alphabets for highway signs that were published ca.1926 . “The new “rounded” style has a more pleasing appearance than the old, and extensive tests have shown it to be consistently more legible.”
These new alphabets were an upper case series with six styles, called Letter Series A through F. The letter forms of each letter series become progressively wider for a given letter height. In addition, the width of the letter stroke, i.e., thickness, also increases with each series. Letter Series A is a narrow stroked extra condensed letter form, and Series F is an expanded letter form with wide stroke.
In 1966, the Federal Highway Administration reprinted the Standard Alphabets in an edition of upper case series B, C, D, E, and F. The Series A alphabet had been deemed “no longer acceptable for use in highway signs” due to poor legibility at high speeds so it was removed from this publication.
Where the previous edition included complex tables of dimensions, the 1966 Alphabets were printed in exact detail for two-inch letter height and placed on 1/4 inch grids to facilitate the enlarging process. A Standard lowercase Alphabet was included in this edition and approved for directional signs on the National System of Interstate and Defense Highways. All names of places and highways on guide signs were to be set in lowercase with initial capitals. These were designed to be used with the capitals and numerals from Series E but with the stroke width widened to approximately one-fifth of the letter height.
In the year 2000 a new specification for using the alphabets made them adaptable current software industry specifications for TrueType and PostScript format fonts. Every character was positioned within a bounding box and assigned a fixed value.
This edition included lowercase letters for all alphabet series and “every effort was made to maintain the same properties of the existing 1966 and 1977 Standard Alphabets”. A uniform stroke width was adopted for all letters and numerals with slight changes were made where necessary to ensure consistency of stroke weight and optical balance from letter to letter. This edition also introduced basic punctuation, and basic typographical symbols for all series (&!”#$¢/*.,:()-@=+?).