The New York Times reports the highway administration said that crosswalk art "diminishes the contrast between the white lines and the pavement, potentially decreasing the effectiveness of the crosswalk markings and the safety of pedestrian traffic."
However, according to the Times, the Ames City Council voted unanimously to ignore the letter.
On Monday, KIRO-TV confirmed that the Seattle Department of Transportation also received a letter, in 2015, from the government about rainbow crosswalks.
"(The letter) informed us that in the future, if we applied for federal funding at an intersection with a multicolored crosswalk they would ask us to remove the markings and replace them with a standard crosswalk marking," Ethan Bergeson, of SDOT, told KIRO-TV.
Bergeson went on to say that so far, none of the 41 artistic crosswalks in Seattle have been affected.
Eleven rainbow crosswalks were installed in Capitol Hill before Pride week in 2015 – costing $66,000.