I personally don't believe that World War I had a great influence on automobile design or inovation. In the United States, the automobile was already increasing in popularity. In the first decade of the 20th century automobiles were certainly seen as luxuries for the wealthy and were considered playthings. However, with the introduction of the Ford Model T in 1908 and the introduction of the assembly line in 1913, the automotive revolution was already underway. Below are some statistics.
Cars Per Person in the United States
1900 1 for every 9,500 people
1910 1/183
1913 1/77
1915 1/50
1917 1/20
1923 1/8
1927 1/4.5
Cars Registered in the United States
1895 4
1900 8,000
1905 77,400
1908 194,400
1909 305,950
1910 458,500
1911 619,500
1912 902,600
1914 1,625,739
1916 3,297,996
1918 5,621,617
1920 8,225,859
Unlike during World War II, the U.S. did not halt the production of automobiles. So car technology continued to progress for the two years the U.S. was at war. The prices also continue to drop however. The role of Ford on the U.S. automobile culture cannot be underestimated, when it was introduced in 1908 the first touring cars sold for $850 ($17,000 in today's money), at a time when most cars cost $1,800 to $4,000. It was a hit and by 1912 22% of cars on the road were Fords. As demand increased the price dropped, so that by 1914 it was $650 ($12,100 today), and $290 by 1924 ($3,200 today). The Ford Model T was popular not only in the U.S. but also in other countries, it was really the car that opened driving to the world. By 1919 70% of all cars in France were Fords, in Italy 85%, in Egypt 99.6%, and in Mesopotamia (Iraq) 99.9%.
As Ford conquered the market worldwide, several new car companies were offering cars in the $600 to $1,000 range already by 1914, among them the Chevrolet which would become the number 2 car in the United States. In Europe cars were not as popular yet, but in Canada, Australia and New Zealand the Ford Model T made the car accessible to the masses as well. In Europe cars like the Citroën 2CV, the Fiat 600 (SEAT 600 in Spain), and above all the Volkswagen Beetle would have the same effect but only after World War II.
Here's a comparison of cars per person in various countries in 1939 to show that in much of Europe the personal automobile was still very much a thing of the wealthy. Suprisingly though, American cars were quite popular around the world with Buicks, Chevrolets, Fords, Pontiacs and Studebakers selling the same models around the world as they did in the U.S.
United States 1/4
New Zealand 1/6
Canada 1/8
Australia 1/9
England 1/18
France 1/18
South Africa 1/24
Sweden 1/29
Norway 1/32
Belgium 1/35
Germany 1/42
Argentina 1/46
Italy 1/93
Spain 1/196
Brazil 1/281
Japan 1/389