Get a used steel framed road bike that can fit wide-ish tires. At least 28mm, 32mm would be great. Check Craigslist for 80's bikes from Japanese brands, or Trek, etc. Make sure you get the right size bike. Get a really good lock and make sure you are using it properly.
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1964 JRJ (Bob Jackson), 1973 Wes Mason, 1974 Raleigh Gran Sport, 1986 Schwinn High Sierra, 2000ish Colian (Colin Laing), 2011 Dick Chafe, 2013 Velo Orange Pass Hunter