It was only a matter of time: Transit agencies are reporting increased ridership due to higher gas prices.

With the national average for a gallon of regular unleaded now at $3.98, motorists across the nation are switching to public transportation. We saw it in 2008, when the national average reached $4, and we’re seeing it all over again.

According to the American Public Transportation Association, $4 per gallon is the tipping point where people begin to drive less and use transit more – a lot more. If gas prices stay this high, we can expect an additional 670 billion transit trips made this year nationwide.

In Boston, transit ridership is up 5 percentoverall, with 7 percent increases on rapid transit lines. Highway and airline traffic are also up, but this is mostly attributed to the recovering economy.

The Gold Line, a light rail line that connects Pasadena, CA to Union Station in Downtown Los Angeles, saw a 10 percent increase in ridership in March 2011 over March of 2010. Transit officials expect even higher ridership as the area’s 12 percent unemployment rate goes down and people head back to work.

It’s not just in big cities with subways and light rail where travel behavior is changing. In Wichita, KS, ridership is up 5 percent despite a recent $.25 fare increase. Wichita Transit expects even more new riders when the average gas price hits $4 a gallon there.

Ridership is up 4.7 percent in Wilkes-Barre, PA in the first quarter of 2011 compared to the first quarter of 2010. The Luzerne County Transit Authority secured a flat $2.56 per gallon price on diesel that will last through June, and they’re replacing older buses with diesel-electric hybrids in 2012.

In McAllen, TX, it’s not just gas prices but alsoflashy new lime green buses and cheap fares that transit officials are giving credit for increased ridership in 2011.

With all this increased ridership nationwide, let’s hope a transit-friendly transportation bill passes. It’s probably not a good idea to slash funding for public transportation when we have so little control over the cost of the alternative.