Traffic Control Signs - a History

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In the period between the World Wars, the United States began its push to standardize road traffic signs and road systems. The two articles reproduced below are taken from the October 1925 edition of The American City Magazine. The first article outlines a series of rules and regulations that were proposed in a report by Herbert S. Swan and George W. Tuttle and were intended to improve the safety and efficiency of intersections. The article describes the “intimate relationship between the arrangement of street intersections and the ease, speed and safety of traffic,” and goes on to suggest minimum and maximum angles for the intersection of roads, as well as other precautionary regulations. The second article deals with the adoption of a standardized road sign system, including the standard Stop and Slow signs that are still in use today. It outlines the shapes, sizes, and colors of the standard road signs adopted by the Joint Board on Interstate Highways in 1925, and provides us with an interesting insight into how faithful today’s road signs are to their predecessors. For an additional article on road sign history, click here.

Street Intersections and Traffic Relief

The intimate relationship between the arrangement of street intersections and the case, speed and safety of traffic is emphasized in the pamphlet on "Land Sub-divisions and the City Plan,” by Herbert S. Swan and George W. Tuttle, published as a supplement to the National Municipal Review for July, 1925. Through the courtesy of Mr. Swan, THE AMERICAN CITY is enabled to publish the accompanying reproduction of a group of his plans showing treatment of street intersections as recommended. The following paragraphs comprise the section on “Enlargement of Street Intersections." From the proposed rules for the guidance of planning commissions, as given in the pamphlet:

No intersecting streets shall be laid out with the angle included between adjoining street lines less than 30° or greater than 150°. Street intersections shall be enlarged by the following minimum amounts;

If the angle included between the adjoining street lines is 90°, then these lines shall be joined by a curve with a radius of 20 feet, the external secant of which is 8.98 feet.

If the angle included between the adjoining street lines is greater than 90°, then these lines shall be joined by a curve with an external secant of 8.28 feet plus 0.27 feet for each degree that the angle is greater than 90°.

This rule relates to street intersections only, it provides for rounding corners and enlarging the area of street intersections so as to facilitate the turning of the traffic into the cross- streets.

Traffic congestion is generally greatest at intersections where the traffic on both streets crosses in the width of one. The capacity of streets is often reduced one-half, and sometimes even more at intersections. It is therefore of the greatest importance to arrange intersections so that traffic will have sufficient room in flow smoothly around corners

To avoid traffic interferences as well as uneconomical and awkward property subdivisions, street intersections of less than 80° or more than 150° (the supplementary angle) are prohibited.

At right -angle intersections the corners are turned with a radius of 20 feet. With sidewalks 10 feet or more in width, ears will have no difficulty in turning close to the curb. The street corner is set back 8.28 feet.

From 90° down to 30° the street corners are rounded with radii decreasing with the angle. The radius of the street corner decreases uniformly for each degree decrease in the angle of intersection until at 30° the radius becomes 12 feet. This allows a width of 24 feet at the corner, which is sufficient frontage for satisfactory development. At the same time, the corner is pushed back 81.36 feet and thereby gives the necessary increased room for turning vehicles.

From 90° up to 150° the street corners are rounded with radii increasing with the angle. The setback from the corner increases uniformly, for each degree increase in the angle of intersection until at 160° the radius becomes 408.96 feet and the setback 54.48 feet.

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Standard Signs Adopted for Federal Highways

The standard signs which have been adopted by the Joint Board on interstate Highways for use on United States highways are of such a character that even a person who cannot read will understand the meaning of those indicating danger by the shape and color. The signs are of two general classes.

One group, the danger and caution signs, will consist of signs of four different shapes representing as many degrees of danger. These will have a yellow background with black letters and symbols. The other group will include the standard route marker in the form of a United States shield and directional and informational signs. All signs in this group will have a white background with blank letters.

Distinctive features of the various signs are outlined as follows in a statement from the office of the Secretary of Agriculture, released on September 5, 1925.

The standard railroad-crossing sign will be round 24 inches in diameter, and will bear the well-understood railroad cross

with the letters RR in black in the upper quadrants on the yellow background.

At point of sufficient danger to require a complete stop, the sign will be octagonal in shape, 24 inches in size, and will bear on the yellow background the word STOP in large black letters. At curves, narrow bridges, steep hills and other points where the degree of danger is sufficient to call for the exercise of a considerable degree of caution but not a complete stop, a diamond shaped yellow sign will be created with the character of the danger described in black letters and symbols. This sign will be 24 inches on each side.

At point of lesser or occasional danger the 24 inch square yellow sign will be used and the necessity for caution will be explained in black letters in a single word, such as SCHOOL, HOSPITAL, CHURCH, etc.

The standard shield adopted as the route marker will be 16 inches vertically from tip to tip. At the top it will bear in black letter on white background the name of the state in which it is erected, and below, in the section of the shield in which the red and white stripes ordinarily appear, will be the initials U. S. and the route number in large numerals.

At the approach to forks or junctions of the interstate routes with other roads, a smaller shield, 9 inches from tip to tip vertically, similar to the route marker but bearing the single letter R or L will be erected below the numbered shield to indicate whether the United States highway branches to the right or left.

Distances along the main routes to cities and towns will be indicated on rectangular white signs with the names of the points and the distances to them and an arrow pointing the proper direction in black.

Distances to points not on the interstate roads will be posted at the intersecting routes leading to such points. The signs, also rectangular, will bear the names of the cities with distances to them in mile in blank on the white background. No arrows will be used on these signs. The side of the road on which they are posted will indicate the direction. In addition to these signs, rectangular white and black signs will be erected at physical features, such as creeks, rivers, lakes, etc, crossed or passed by the interstate roads; and the traveler will be notified at points where the speed limit is changed by a standard black and white sign prescribing the speed limit.

In the series of caution and danger signs the degree of danger is signified by the shape as well as the wording or marking of the signs. A round sign will be used only at railroad crossings; an octagonal sign will be used only at points where a complete stop is demanded by extreme danger ahead; the diamond shaped sign will always indicate to the driver of a motor vehicle, even though he be color blind and unable to read, that there is some road condition ahead which calls for the exercise of caution. The square sign will invariably indicate the need for a lesser degree of caution.
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Yellow is the color of danger or caution. Wherever the motorist sees yellow be will know there is some degree of danger ahead. The white signs will give him information and guidance, but will not call for caution.
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The system of interstate highways on which these signs will be erected was designated recently by the joint board of state and Federal road officials. A committee of the board is now working on a numbering scheme and it is expected that maps of the routes selected with the numbers indicated will be available in a reasonably short time.

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1925 Articles from American City Magazine

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