It's not that the city that clearly does not care much about radiation safety (cesium beef for kids, dumping radioactive ashes in the ocean) did the measurement on its own. A citizen measured the radiation in the area, got the sample and had it tested using his own money, and alerted the city when the result came in.
Then, the city finally went and took the sample to be tested. The result was 42,000 becquerels/kg of radioactive cesium from the dirt that accumulated around the drain grid on the side of the road in Kohoku-ku (Okurayama) in Yokohama City, just inside the 250-kilometer radius from Fukushima I Nuclear Power Plant.
The city also found 35,000 becquerels/kg of cesium in the sediment at a fountain nearby, 27,600 becquerels/kg from the dirt overflow from the plant box on the road, and 11,320 becquerels/kg from the dirt overflow from the plant box on the side walk.
Here's Yokohama City's press release, taken from the message board of Yokohama Assemblyman Ota:
By the way, Assemblyman Ota suggests that Mayor Hayashi is known within the city administration as someone who doesn't think on her own (i.e. she relies on others in any decision making). That makes sense. That's been the ideal for the head of any organization in peace time in Japan, to be the figurehead. Literally deadly in the time of crisis like this.