Siim was born in Germany, lived in Estonia during the German occupation (1942 – 1944) and immigrated to the US in 1950. He moved to Chicago in 1970 when he joined the UIC faculty to teach and develop courses in computer mapping. He was the Head, Department of Geography; Executive Director, Urban Transportation Center and President, Illinois Universities Transportation Research Consortium.
He has taught courses at the University of Illinois at Urbana Champaign, University of Wisconsin-Milwaukee and Chalmers Technical University (Göteborg, Sweden) and has guest lectured in Korea, Taiwan, China, Germany and Estonia.
He has authored nearly one hundred articles and reports. He has been the principal or co-principal investigator on over fifty funded research studies.
He is Vice President of the Estonian House of Chicago, on the board of the U.S. Baltic Foundation and is Honorary Vice Consul of the Republic of Estonia.