“I have been building and refining the H3 detector over the last eight years of experimentation. I started out building analog machines that weighed 200 lbs. This version of the detector is the fourteenth iteration of the device and it weighs less than 10 ounces. . . . The next version of the H3 detector will feature full mapping through GPS.”

Christensen is also keeping the device open to other markets and devices: “I am now designing an ASIC chip set (Application Specific Integrated Circuit) that will be a miniaturized engine with APIs. . . . The APIs will be open to the world of designers so they can use the engine in any application and integrate this engine with existing devices, like the cell phone, weapons systems, and other hand held devices.”

But trucks and tractor-trailers come here all the time, as they do in similarly inappropriate spots across Britain, directed by G.P.S. navigation devices that fail to appreciate that the shortest route is not always the best route.

“We map the reality — the streets, the signposts and the road infrastructure as it is in reality,” said Dirk Snauwaert, a spokesman for Tele Atlas, which provides digital maps to portable navigation systems. “We cannot change that reality in our database. Who are we to make a change and say, ‘You cannot drive in that road’ if, in reality, you can drive in that road.”

Mr. Snauwaert said it was up to local communities to make it clear what roads were not appropriate for trucks, and to install signs saying so. The relevant information, including things like height, width and weight constraints, could then eventually be integrated into the databases used for G.P.S. devices, he said.