Unless the still shot below has been photo-shopped, the object appears to be a flag.
Reports are the driver was a fan of Adolph Hitler.
So, members of one side admired a mass murderer like Hitler, while large numbers of the other no doubt admired mass murderers like Mao tse Tung. ANTIFA is a combination anarchist - communist organization. The red and black symbol is the logo for Anarcho-syndicalism.
The following is from ANTIFA’s website, New York City chapter...
About us
Antifascists and Anticapitalists!
https://nycantifa.wordpress.com/about-us/
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With the red and black anarchist signs and logos they appear to be anarco-syndicalists, a combination of communist and anarchist.
Again from their website (violence is CLEARLY promoted in the banner below)...
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Anarcho-syndicalism
A common Anarcho-Syndicalist flag.
In the early 20th century, anarcho-syndicalism arose as a distinct school of thought within anarchism.[78] With greater focus on the labour movement than previous forms of anarchism, syndicalism posits radical trade unions as a potential force for revolutionary social change, replacing capitalism and the state with a new society, democratically self-managed by the workers.
Anarcho-syndicalists seek to abolish the wage system and private ownership of the means of production, which they believe lead to class divisions. Important principles include workers' solidarity, direct action (such as general strikes and workplace recuperations), and workers' self-management. This is compatible with other branches of anarchism, and anarcho-syndicalists often subscribe to anarchist communist or collectivist anarchist economic systems.[79] Its advocates propose labour organization as a means to create the foundations of a non-hierarchical anarchist society within the current system and bring about social revolution.
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YouTube Video:
The O'Reilly Factor confronts Bill Ayers:
October 24, 2008:
(note the red communist star, and anarcho-syndicalist red and black, on his shirt)
http://www.youtube.com/watch?v=dP3uvK9gTIY
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Bill Ayers: "I considered myself partly an anarchist then and I consider myself partly an anarchist now. I mean, I'm as much an anarchist as I am a Marxist which is to say I find a lot of the ideas in anarchism appealing."