As a psychiatrist who specializes in treating ADHD, I've become accustomed to my patients arriving late for their appointments. This is common not only because of unpredictable traffic conditions, which are common, but also because many people with ADHD have difficulty with time. It seems that our "internal clocks" which keep track of the passing of time, allow us to estimate how long something will take us to do, and contribute to general time awareness are integral parts of what we refer to as executive functioning. In addition to organization, prioritization, initiation and completion of tasks, the part of the brain responsible for keeping time is either on or off for people with ADHD. It's not uncommon to become engaged in an activity, look up at the time, and be surprised at how much has gone by. Similarly, it can seem reasonable to fit in "one more thing" before leaving for an appointment, only to discover that there really wasn't enough time to get it done and be on time.
Understandably, many people without ADHD find this annoying, or worse, offensive. Being late seems to make the statement "My time is more valuable than yours." For my patients, however, nothing could be further from the truth. They don't intend to be late. In fact, they really want to be on time, and feel guilty and ashamed when they're late. It's hard to explain chronic lateness to others, as it's easily passed off as a character deficiency.
There are many techniques and tricks to help alleviate this problem, most of which involve "tricking yourself" into being early (which usually means on time), making a conscious effort to document how long certain things take, and externalizing your internal clock as much as possible through the use of alarms, calendars and multiple reminders.
The most important thing to remind yourself, of course, is that it's progress, not perfection, when it comes to time.