The six electric vehicles known collectively as Little Roady navigate a 5-mile loop in regular traffic and steer themselves into 10 curbside stops, giving lawyers and laborers alike a free ride.

Of all the freeways in the United States, the most productive corridor for self-driving trucks appears to be Interstate 5 through Washington and Oregon, according to a report released this week by the Kirkland, Wash.-based Inrix data company.

By several standards, the transportation network of Washington state is creaking under the pressures of age and economic growth.