Washington state's Transitional Bilingual Instruction Program (TBIP) has existed since the passage of Senate Bill 2149 in 1979, and TBIP is codified in the Revised Code of Washington (RCW) 28A.180. The bill, called "The Transitional Bilingual Instruction Act," recognized that "classes which are taught in English are inadequate to meet the needs of" English learners (ELs). Because of the bill, students who need bilingual instruction can receive it while they become proficient in English. RCW 28A.180.090 requires the Washington Office of Superintendent of Public Instruction (OSPI) to measure increases in the English and academic proficiency of students who are eligible for TBIP services. OSPI also tracks the academic progress of former TBIP students throughout their K-12 career. During the 2016-17 school year, 135,159 students were identified as ELs, a 3.7 percent increase from 2015-16. Enrollment was highest in urban areas along Interstate 5 and the Yakima Valley. ELs are typically eligible for TBIP services for about three years. Each year, TBIP students take the English Language Proficiency Assessment for the 21st Century (ELPA21). When a student passes the annual ELPA21 assessment, they exit TBIP. About one out of every eight ELs (13.6 percent) who took the ELPA21 in 2016-17 earned a Proficient-Level 3 score, allowing them to transition out of the TBIP. About 73 percent of ELs achieved a Progressing-Level 2 score and 11.5 percent scored as Emerging-Level 1. Students with scores of Emerging-Level 1 and Progressing-Level 2 continue to receive TBIP services. Students served by TBIP in 2016-17 spoke 225 different home languages. The most identified language was Spanish, followed by Russian, Vietnamese, Somali, and Arabic. Total expenditures to support English language development services across the state was $154.4 million, of which $123.6 million was from TBIP funding. This was a 9.93 percent increase in the TBIP total expenditures from the previous year.