I could only found US transportation GHGs from 1990 to 2004 -- a person can go crazy searching for the exact data -- available here. I extrapolated to 2006 -- I'm not going to go through the details -- and got 1,985.5 MMT CO2e for the US Transportation sector GHG emissions in 2006. Dividing by the 2006 population given here, we get
per capita US transportation GHG emissions, 2006 = 6.6 MT CO2
That compares to Portland's claim of 5.6 MT.
So Portland is claiming that its per-capita transportation emissions are 15% below the national average.... Is that reasonable, for a city?