Vanguard Group Inc., the world’s largest mutual-fund company, is calling on regulators to conduct a test program of a rule that would force most U.S. stock trades onto public exchanges.

By sending orders away from venues such as dark pools and into the public markets, the rule, known as trade-at, would help share price discovery and improve investor confidence, Joseph Brennan, principal and head of the global equity index group at Vanguard, said in remarks prepared for a Senate hearing today.

“Publicly displayed liquidity is the foundation of a transparent and efficient market,” Brennan said in his statement. “A structure that encourages publicly displayed limit orders reduces spreads, increases liquidity, promotes price discovery, and lowers transaction costs for all investors.”

Brennan is due to speak at a hearing on U.S. stock markets convened by the Permanent Subcommittee on Investigations.