Final answer:
Transportation Secretary Buttigieg referred to the discriminatory impact of highway construction on marginalized communities, particularly communities of color.
Step-by-step explanation:
Transportation Secretary Buttigieg acknowledged the 'racism physically built into some of our highways' as a reference to the discriminatory impact of highway construction on marginalized communities. The construction of the interstate highway system in the mid-20th century often resulted in the destruction of urban neighborhoods, particularly impacting working-class communities of color.
Highways were often built through these neighborhoods, displacing residents and further segregating cities. This had long-lasting social and economic consequences, as it facilitated suburbanization for white Americans while leaving economically disadvantaged African Americans behind in urban cores.