Rules of the Road

When does putting a small motor on a bicycle make it a moped in the eyes of the law?

Question: I am considering putting a small gas motor kit on a mountain bicycle. Does it then become a moped? Does it require a license? Can It be driven on sidewalks? Or bike paths? Or freeways? Any other information on this?

I have been looking online and I get conflicting information.

Answer: That sounds like a fun summer time project, but if you’re planning to ride it on public roads, I hope you haven’t started yet. I don’t think you’re going to like the answer, or answers really, given the number of questions you were able to cram into 50 words.

I’ll tackle each question separately, and by the end you should have a good idea of what it’ll take to make your project legal.

Does adding a gas motor to a bicycle make it a moped?

Sort of, but not a legal one. Once you put a gas motor on a bike, you’ll have to make it comply with the Federal Motor Vehicle Safety Standards (FMVSS) that are applicable to a motor-driven cycle. Unless you’re a vehicle manufacturer or a complete traffic nerd, you’ll probably never read the FMVSS, so I’ll provide the relevant highlights.

The federal government thinks of mopeds as wimpy motorcycles, so they have to comply with most of the motorcycle safety requirements. This includes a headlight, tail light, brake light, license plate light (yep, you’ll need to license it), mirrors on both sides of the bike, a horn, a muffler and possibly brakes. And each of those items has its own standards it has to meet.

Also, almost all of those items require electricity, so you’ll be designing and installing an electrical system for your bike.

Does it require a license? You don’t need a motorcycle endorsement to ride a moped, but you do need a valid driver license. That’s assuming the motor you add is less than 50 cubic centimeters.

If it’s bigger than that it gets bumped into the motor-driven cycle category, and that requires a motorcycle endorsement. Keep it under 50 cc or sign up for motorcycle school.

The bike though, that’s a different story. As mentioned earlier, mopeds must be registered and display a license plate. Normally registering a vehicle involves a title and a bill of sale. But since you’re building it you won’t have either of those, so you’d need to apply for a certificate of title and get a homebuilt vehicle inspection from the Washington State Patrol.

If you were to do all the above listed tasks, you’d now have a moped you could legally ride in Washington.

But you couldn’t ride it on the sidewalk, you couldn’t ride it on a bike path, and you couldn’t ride it on the freeway.

Once you add up all the costs in both time and money, along with the limitations that come with building your own moped, it begins to seem like it’s not worth it.

By this point you might protest, “But I’ve seen plenty of bikes with gas motors riding around.”

What you’ve probably seen are plenty of illegal mopeds riding around. The companies that sell the gas-powered bicycle engine kits know it too. If you look at the fine print on their products, you’ll find something like this: “Upon purchase, the buyer agrees to use products for closed-course riding only and not for public roads.”

If you’re really set on motorizing your bicycle, you might consider an electric motor instead. In what could be perceived as discrimination against gas-powered bicycles, adding an electric motor to a bicycle has none of the hurdles required for mopeds.

As long as your electric bike is limited to 20 mph, it can ride almost anywhere that a human-powered bicycle can ride. You can ride it on the sidewalk (with some limitations), you can ride it on the freeway (with the same limitations as non-electrified cyclists), and you can ride it on a bike path (unless it’s posted as for non-motorized vehicles only).

You don’t need a license, you don’t need to register it or get an inspection and you don’t need to add that list of stuff specified by the FMVSS.

Whether you follow through on the moped plan, switch to an electric bike, or continue with human power, don’t forget a helmet. State law only requires it on a moped, but this is another example of the difference between the law and best practice.

Be safe and enjoy the ride.

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Doug Dahl, Target Zero Manager for the Whatcom County Traffic Safety Task Force, answers questions about road laws, safe driving habits and general police practices every Monday. Ask him a question using our form. Target Zero is Washington’s vision to reduce traffic fatalities and serious injuries to zero by 2030. For more traffic safety information visit