I won't even try to top Marylandnewbies' post. It say all the basic stuff so elegantly. I only want to comment of the fringe stuff, the freakish emergencies sort.
On my regular routes I have memorized the locations of all accessible port-a-potties and public toilets (McDonalds etc). More than once this knowledge has prevented an accident. Also god to learn if you need to add or remove a layer during rain-wind-snow.
Got your tools, spares tubes 2 0r 3), cell phone, ID, money. A short commute you can walk home or to work. Not likely with 18 miles. Secure parking at work. Try hard to get inside parking. Companies and/or building owners need to be approached very carefully about this sometimes. Outside check threads on locks and check out
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