ROME — Via dei Fori Imperiali, a multilane artery running through the heart of Rome, is typically a frenzy of swerving Vespas, zipping Smart cars and honking Fiat taxis.

But Mayor Ignazio Marino is seeking to transform the avenue to something calmer, where Gucci loafers and sensible sneakers would rule.

Mr. Marino’s plan to ban private traffic on the roadway, which bisects a vast archaeological site, from the central Piazza Venezia to the Colosseum, has prompted grousing and histrionic debate over a project that conservators say would solidify the world’s largest urban archaeological area.

This being Rome, the first high-impact initiative of his seven-week-old administration, which goes into effect on Saturday, has provoked its share of unfavorable comparisons with the overweening ambitions of emperors past. “The mayor’s job is not to pass into history, but to work for his citizens,” said Luciano Canfora, a professor of classics at the University of Bari. “We already had Nero, that’s more than enough.”...