The design was actually developed during the 1931 congress, by a sub-committee.
Early traffic signage was mostly confined to warning signs for motorists. The 1908 World Road Congress started out with three signs, until 1926 three more were added. Scandinavian countries had even argued that only one sign, the upright white triangle with a red border, was enough.
The first international treaty mentioning prohibition signage was the 1931 Geneva Convention Concerning the Unification of Road Signs, negotiated by League of Nations Transit and Communication Committee. While the 1926 Paris Conference had only adapted a group of six warning signs for motorists, now a whole panel of signs, grouped into I. Danger Signs, II. Signs giving definite instructions and III. Signs giving indications only were introduced.
In preparation of the congress, the Permanent Committee on Traffic had prepared a Draft Convention concerning the Unification of Road Signals (English: pp. 32–34). It contained (p. 39) a round blue sign with a red border, but no diagonal stroke, described as
(e) Sign prohibiting waiting by vehicles. — This sign denotes that waiting is prohibited on the side of the road where the sign is placed (see Figure 16, Table III).
Somehow, during the proceedings at the Geneva conference, this sign became the subject of a controversy. Seemingly nobody (with the exception of the Italian delegate, where the sign had already been adopted) found it satisfactory. The Netherland delegation proposed some different signs (pp.278–283), and it contained another "Waiting prohibited" sign: a round white sign with a red border and the black letter P in the middle, again without a diagonal stroke. While other parts of their proposal was dismissed, this variant made it into the second draft of the convention, to the discontent of some other delegates.
Comparison of Committee and Netherlands proposals
Finally a sub-committee was set up to find a compromise (p. 271–272):
Held on March 25th, 1931, at 3.30 p.m.
Appointment of a Sub-Committee
The question of the sign to be adopted for "No Parking" and "No Waiting" was referred to a sub-committee consisting of M. Pflug, M. Le Gavrian, M. De Schulthess, M. Schneider, M. Schönfeld, M. Persyn and M. Rothmund.
Section (f) of the Annex (waiting prohibited) was therefore reserved.
Held on March 26th, 1931, at 3 p.m.
XXIV. Report of the Sub-Committee appointed to submit Proposals concerning a Sign prohibiting Waiting and a Sign prohibiting Parking.
The Secretary-General of the Conference explained that the Committee thought it necessary to use two different signs—one indicating “ parking prohibited ” and the other “ waiting prohibited ”, it being understood that the sign “ waiting prohibited ” could be used in cases where both parking and waiting were prohibited.
The final description of the signs as adopted by the committee and included in the convention reads:
f) Waiting prohibited: This sign shows that waiting is prohibited at the side of the public road where it is placed. The centre of the disc must be blue surrounded by a wide red border with a diagonal red stroke (figure 9, Table II). It may bear inscriptions giving information as to the hours during which waiting is prohibited, etc.,
g) Parking prohibited: Red disc with circular centre in white or pale yellow bearing the letter P. with a diagonal red stroke (figure 10, Table II).
And lo and behold, there appears the diagonal red stroke:
The seven members of the sub-committee were:
- Dr. F. Pflug, engineer, ministerial councillor at the Ministry of Communications, Germany
- P. le Gavrian, Inspecteur-General of roads and bridges, France
- G. de Schulthess, Director of the Union of Swiss Towns, International Union of Towns and Local Authorities
- Franz Schneider, engineer, architectural adviser to the municipality of Vienna, Austria
- G. F. Schönfeld, Administrator at the Waterstaat, Netherlands
- A. J. Persyn, Chief of the Traffic Department at the Ministry of Public Works, Belgium
- Henry Rothmund, Chief of the Police Division of the Federal Department of Justice and Police, Switzerland
Frank Schipper: Unravelling hieroglyphs : Urban traffic signs and the League of Nations, Metropoles 6 / 2009 describes the context of the conference. He points out that only from 1927 onwards, representants of cities were present and made their interests known.
No sooner had the ink dried on the Conventions of Paris than the Committee continued to identify traffic signs as the most urgent matter on its agenda. The Committee now introduced an explicit urban dimension by inserting a distinction between urban and non-urban signs in its work. A sign of the rising importance of the urban was the presence of Dr. De Schulthess on behalf of the International Union of Towns (IUT) at the Committee's fifth session. De Schulthess admitted that the IUT had long ignored the existence of a Road Committee, and had cooperated exclusively with the AIACR and PIARC. A committee representing local Swiss authorities as well as automobile, biking and touring interest associations had just drawn up a set of urban road traffic signs across Switzerland to be presented to an IUT meeting in July.² The collection included signs prohibiting passage, indicating one-way traffic or a compulsory direction, or authorising the parking of vehicles. The Road Committee carefully considered the outcome, which had in its turn been inspired by earlier deliberations taking place within the Road Committee itself.
² De Schulthess to League of Nations, 27 April 1927, box R1132, League of Nations Archives (LoN).
So the trace seems to point to Swiss communities and the 1920ies. The UCLG, successor of the IUT, mentions in its centenenary publication a 1927 congress in Bern dedicated to road and traffic signs. But it seems that none of these predecessors used the diagonal stroke. The minutes of the 5th session of the LoN Traffic Committee (November 1927) describe a "blue disk with a red border". Looking into the proceedings of the 1930 World Road Congress in Washington, there are three country reports that mention the work of the IUT conference and the LoN Traffic committee, and they all describe a "blue disk with a red border": France, Italy, Switzerland.