Our intervention step of the Urban Probe involved strategically placing our own “trash” or traces of people across the city. To understand the individual level of curiously in accidentally discarded traces of fellow city inhabitants, we adapted Milgram’s “Lost Letter Technique” into a new methodology we call the “Lost Postcard Technique”.

Milgram’s original study involved dropping hundreds of seemingly lost letters (addressed and stamped) in different neighborhoods. The letters were addressed to political and often extremist groups. By examining the proportion of returned letters (i.e. those forwarded on by individuals that happened across them within the city), he was able to assess public opinion concerning issues associated with the groups the letters were addressed to. Rather than actively soliciting direct answers from individuals, this technique allows anonymous urban group behaviors and attitudes to be measured passively by tallying the ratio of returned letters across several variables.

Our goals for the Lost Postcard Technique were:

To what degree are people interested in traces left by others? Will the items create narratives?

What interpretations of value do city dwellers place in the detritus found within the street?

Will found items be viewed as lost or trash? And will people take responsibility for this “lost trash”?

Will the levels of interest, responsibility and curiosity vary depending on the location, and context, and personal nature of the lost item?





To reduce our chances of being observed, at 5:30am we began dropping 110 hand-written, addressed, and stamped postcards on the streets of central San Francisco. The cards were distributed as evenly as possible across a wide area of the central city. The cards were divided into three categories distributed in equal proportions and designed to be decreasingly personal, in content and intended audience – card types A, B and C

All the cards contained a short message discussing a recent event shared between sender and recipient, each ending with a unique URL which contained a link to online images from the fictions event. For example, the personal cards (Type A) were each handwritten with the text:


Got this picture back and thought of you. What a blast last week! I still can’t believe we didn’t get caught sneaking onto that roof. Keep these pictures to yourself.       - Chris

The URL provided the opportunity for deeper investigation into the card’s content, a secondary level of recordable action that would further reveal levels of curiosity. Each of the 110 URLs had a different numerical code; these codes were used to log the time, location and context of the dropped cards. This allowed us to track the specific actions taken for each card in one of four expected outcomes:

  • no recordable action – card either disregard as trash or picked up and disposed/ignored

  • card picked up and returned via mail

  • card picked up and URL visited

  • card picked up, URL visited, and returned via mail

The cards were also placed in a 5 different types of context:

on the street
on raised surfaces such as benches, steps, window sills, newspaper boxes and on top of trashcans
on car windshields
on bike or moped baskets to sample what reaction there would be to cards found on private property within public space
on public transportation such as BART and cable cars within the city


Breakdown of actions taken for all 110 postcards


Type of cards with added messages written on returned card

Within 3 days, we had received nearly 49 of the postcards (45%) in the postal mail – forwarded on by individuals. Exceptional cards went on long journeys from a bench in a Chinatown park to Reno, Nevada, from where it was returned 19 days later. The proportion of returned cards across the 3 categories was fairly consistent, as were the results across the 5 contexts. It soon became apparent that the quality of the study would lie in the richer actions and stories attached to the minority of the found cards.
We received 4 cards with added messages, something we had not anticipated. Two of these “message” cards noted that the card had been found – one with contact details.

The fourth card which was a personal rooftop card (Type A) left on a concrete bench on Columbus Avenue in North Beach at 8:53am was returned with the added message:

Actually, we have video footage of you sneaking around, we turned them over to Tom Ridge. Vote Kerry!

There were only four URLs hits in total. Three were an investigation of the story contained in the personal cards; one of these cards was kept for six days until the URL was hit and a further three days before the card was finally returned. Two of the URL card hits were never returned via mail. Although it is hard to draw clear quantifiable relationships from our results, it would be fair to assume that a sizable section of urbanites (reflected in this study) are inspired into curiosity and more explicit action by more personal traces left by others in the urban spaces they share – while the majority may have passed by the lost cards as just another piece of trash within the city.

Actual scans of all three postcard types:

Card Type Card Front Card Back


Urban Atmospheres at Intel Research