New technology can be a wonderful thing for travelers.

Siri, my electronic iPhone pal, has helped me successfully navigate countless miles of unfamiliar highway. She also has tried to drown me in the Mississippi River. But that’s a story for another day.

Today’s good/bad technology topic is electronic toll roads, which allow drivers with transponders to easily pay tolls without stopping, but which can also leave rental-car drivers with unexpected charges.

Electronic tolls have been around for years. In the past, it has been easy for travelers who don’t have a toll transponder to simply pick the “cash” toll booth, pay the toll and be on their way. But more and more toll roads and bridges across the United States have eliminated toll booths and, with them, any option to pay on the spot.

In most places where toll booths have been eliminated, cameras scan the license plates of drivers without transponders. The owner of the vehicle gets a bill in the mail, usually a bit higher than the toll for those using transponders.

But when the owner of the vehicle is a rental-car company, those companies can tack on their own fees — sometimes quite high ones — and forward the bill to you.

Avoiding the tolls — and fees — can be tricky. Several times I’ve come upon electronic tolling stations with no warning and no chance to exit the highway to take a toll-free route.

I ran into an unexpected toll recently while driving on the Massachusetts Turnpike from Boston to Springfield. My rental-car company didn’t mention tolls when I picked up my car, but on the highway, I found myself passing through a quick succession of toll scanners with no option to pay cash. The total toll of about $6 more than doubled after my rental-car agency added its fees.

I got off lucky. On a recent trip to California, a co-worker saw her rental-car company charge $73.72 for $13.72 in electronic tolls incurred. Ouch.

Every rental-car company has a different fee schedule for electronic tolls, and the fees can vary by state.

The fees can be modest. In Florida, which has many fully automated toll roads and bridges, National, Alamo and Enterprise automatically charge $3.95 a day (plus tolls) for each day a driver incurs a toll. There is no need to sign up beforehand, and the companies charge no extra penalties.

At the opposite end of the spectrum is Dollar and Thrifty, which charge a flat $10.49 a day for every day of your rental contract — if you sign up at the rental counter. Otherwise, you’ll be charged an extra $15 per toll, up to $90. And, speaking from experience, I can say that not every agency informs you of the option to pay beforehand.

So be sure to check with your travel agent or rental-car company about possible tolls along your route. And always ask at the rental-car counter about toll policies and possible penalties.

Another option for frequent travelers is to buy a toll transponder, which can be carried along in a rental car. The E-ZPass transponder that works on the Ohio Turnpike also works in 14 other states, among them Pennsylvania, New York, Illinois, Indiana, Kentucky and West Virginia. Drivers can rent an E-ZPass from the Ohio Turnpike Commission for 75 cents per month. Visit ezpassoh.com.

Florida has its own system called SunPass. Travelers can buy a SunPass transponder at many stores in Florida, including CVS pharmacies. Permanent SunPass transponders cost $19.99. “Mini pass” window stickers, which can be used on only one vehicle, cost $4.99. As with the E-ZPass, funds must be added in advance to a SunPass account to pay tolls. Visit sunpass.com.

— Steve Stephens can be reached at sstephens@dispatch.com or on Twitter @SteveStephens.