If you’ve had a hard time filling your school bus driver roster recently, you’re not alone. The past couple of years have yielded widespread driver shortages for school districts and contractors alike.

School bus driver shortage is a problem that wanes and waxes, and 2014 and 2015 were decidedly on the waxing side.

While a variety of factors at the local level — driver pay, department morale, job market competition, etc. — affect whether a school bus operation is able to recruit and retain enough drivers, a reliable indicator of driver shortage on the national level is the unemployment rate.

Over the years, our research has consistently found that the prevalence of school bus driver shortage is high when the unemployment rate is low, and vice versa. It seems that when more jobs are available, fewer people are willing to get behind the wheel of a yellow bus, or they are drawn to better-paying gigs.

For example, let’s look back at 2007, when the U.S. unemployment rate was just under 5%. That year, School Bus Fleet’s School District Survey found that 85% of the districts surveyed had some degree of driver shortage.

In 2009, with the recession in full swing, the national unemployment rate soared to 10%. Accordingly, our 2009 School District Survey recorded a huge decrease in school bus driver shortage: Only 58% of the districts surveyed then were experiencing driver shortage — down from 85% just two years earlier.

In the past few years, as the unemployment rate has declined, school bus driver shortage has gone back on the increase. By 2015, unemployment had fallen to 5%, and our District Survey showed that 92% of respondents had a driver shortage.

While driver shortage has gotten extensive coverage here in the pages of School Bus Fleet, it was telling to see that the issue even attracted the mainstream media’s attention in 2015. In the span of a few months, I was interviewed about school bus driver shortage by reporters for several major news outlets: NPR, CBS News and the Minneapolis Star Tribune.

The NPR piece, which aired nationally on the “All Things Considered” program, focused on the severe driver shortage that has been hampering Metro Nashville (Tenn.) Public Schools.

Officials for Metro Nashville, which operates more than 600 school buses, said in October that an unusually high number of driver resignations had created close to 150 openings. The district developed a new plan to address driver recruitment and retention. The key changes are pay raises, a guaranteed 40-hour work week and attendance bonuses for the district’s drivers.

As we’ve shown, school bus driver shortage tends to be a greater challenge when unemployment is low. When the job market heats up, school bus operations have to make sure that they’re offering competitive compensation to avoid getting left in the cold.