TriMet's fleet uses diesel motors for propulsion, the agency has also experimented with hybrid electric buses. The first two hybrid buses entered service in 2002, but in 2008 TriMet stated that the buses had not performed sufficiently better than its newest diesel buses to justify the estimated 50-percent-higher purchase cost, and that consequently the agency had no plans to purchase additional hybrid buses at that time. These first two hybrid buses were retired in 2012. However, with hybrid technology having improved since that earlier purchase, TriMet acquired four new hybrid buses in 2012 and placed them into service in January 2013 on line 72, a long, mostly level north–south route. TriMet acquired four more hybrid buses in 2015 with even greater electronic technology on board. Since October 30, 2006, all TriMet buses and paratransit minibuses have been fueled by a B5 biodiesel blend. Plans to increase to a B10 or higher mix were later put on hold as a result of cost increases and problems experienced in a trial use of B10 blend in about one-quarter of the fleet. The agency delayed new bus purchases for four years due to the recession of 2008 and the resulting decrease in income from taxes. In 2012 TriMet began to replace buses on an accelerated schedule. By that time some of the buses in the fleet were more than 20 years old and had become more expensive to maintain and less reliable. The first order of 55 40-foot Gillig buses began to enter service in fall 2012, followed by 70 buses in 2013, 60 in the summer of 2014, followed by another 30 in October 2014. In early 2015, TriMet received its first new 30-foot buses in more than 20 years. These 22 Gillig buses are similar to the rest of TriMet's new buses, but their shorter length allows them to serve routes with tighter turns and difficult terrain. By mid-2016 TriMet planned to have 326 buses on the road under four years old, which would allow the agency to meet the industry standard of an average age of eight years. TriMet's 3900-series buses, an order of 64 diesel buses built in 2018 and 2019 and entering service in February 2019, introduced a new paint scheme for the agency, of all-over blue with a trio of semi-upright orange stripes near the rear on each side. The Frequent Express service operates a fleet of