Municipal bond exchange traded funds have been on a tear, and the munis market still looks relatively cheap compared to U.S. Treasuries.

The iShares National AMT-Free Muni Bond ETF (NYSEArca: MUB) has increased 8.8% over the past year. [Diversify with a Cheap, Broad Munis Bond ETF]

Benchmark 10-year muni yields dipped 0.28 percentage point to a 20-month low of 1.83% this month, one of the steepest declines since January 2014, Bloomberg reports. Bond yields and prices have an inverse relationship, so a falling yield corresponds with higher bond prices.

MUB has a 6.37 year effective duration and a 1.54% 30-day SEC yield, or a 2.73% taxable equivalent 30-day SEC yield.

Nevertheless, muni debt is close to their cheapest since December 2013 relative to Treasuries. Looking at the rates on state and local bonds relative to federal debt, the ratio of interest rates hit 106 percent Thursday, which suggest that municipal bonds have weakened relative to the federal debt. Historically, the muni-to-Treasuries ratio has been below 100% since interest on state and local debt is tax-exempt, which make the assets more attractive for high-income investors.

Yields on benchmark 10-year Treasuries touched 1.756% Thursday. The iShares 7-10 Year Treasury Bond ETF (NYSEArca: IEF), which has a 7.73 year duration and a 1.83% 30-day SEC yield, has increased 11.7% over the past year.

“Despite the performance in munis over the last 10 days, it’s lagged Treasuries,” Adam Buchanan, vice president of sales and trading at Ziegler, said in the Bloomberg article.. “There’s still room to run here.”