Years ago in a life-drawing workshop, the instructor said something that has stayed with me ever since. Mark Kang-O’Higgins approaches every pose, regardless of duration, as if it were only 30 seconds long. Even if he knows the pose will last 20 minutes (or two hours or several weeks, for that matter), he captures the entire gesture in half a minute. Then he spends the rest of the duration shaping and refining that initial gesture. Finally, at the very end of the duration, he puts in details. That way, no matter how much time he has, he will always have captured the most essential part of the pose.
The method he describes is basically what I practice whenever I do life drawing, but it is just as applicable when urban sketching, too. When I spotted this mail truck, I didn’t know if the carrier would return in 30 seconds or 15 minutes, so I captured the truck’s “gesture” (if a mail truck can be said to have one) as quickly as possible – probably less than a minute. Then I marked the essential shadows and other dark areas. For the remaining time (which turned out to be a leisurely 20 minutes or so), I continued to refine the shapes and put in details. I was completely done with the truck and already working on the willow when the carrier finally appeared. It turned out he was inside the truck all that time. He came out to deliver one package, then drove off. But even if the truck had disappeared after five or 10 minutes, I think I had enough on the page that I could have finished it. This method is especially useful when sketching people, who tend to walk off at the most inconvenient times.
Come to think of it, this lesson that has stayed with me is a version of the same advice I’ve heard from nearly every urban sketching instructor or author: Start with the large shapes first, then get progressively smaller and more detailed as time allows.