A "standard" modern highway lane is 12 feet wide (1.66 inches in HO scale) You could use lanes as narrow as 10 feet (1.38 inches).A median island could be anywhare from about 1-foot wide (0.14 inches) to the same width as the lanes or more. Dont forget outside shoulders, typically 4 feet to 8 feet wide.
The link below is to a Caltrans (Calif Dept of Transportation) manual that shows some typical Highway striping. While rasied medians are not shown, the layouts with raised medians are similar.
http://www.dot.ca.gov/hq/traffops/signtech/mutcdsupp/pdf/CA-Chap3B.pdf#xml=http://dap1.dot.ca.gov/cgi-bin/texis/webinator/search/pdfhi.txt?query=pavement+markings&db=db&pr=www&prox=page&rorder=500&rprox=500&rdfreq=500&rwfreq=500&rlead=500&sufs=0&order=r&cq=&id=455eb18f19
I tried to sell my two cents worth, but no one would give me a plug nickel for it.
I don't have a leg to stand on.