Get ready to see more electric bikes on NYC streets: Citi Bike announced a major expansion of its fleet of pedal-assist e-bikes, with 4,000 of the speedier cycles rolling out over the next few months.

The downside: Using one of the e-bikes will cost an additional $2, or an additional 50 cents for those who qualify under the reduced fare program. Currently, it costs $3 to take a 30-minute trip on one of bike-sharing services normal two-wheelers. And even those who have a Citi Bike membership won’t get off scot free; the $2 surcharge applies to them as well. The extra cost is a necessary evil: Citi Bike will need to hire extra employees to keep up with the demand for e-bikes, whose batteries need to be replaced every 45 minutes, as well as maintenance costs.

E-bikes have proven very popular since they first landed on city streets last summer, so much so that it was hard to find one in the early months of the roll-out. With thousands more hitting the road by June, Citi Bike sees them not just as a last-mile transportation option, but new, dedicated mode of transport.

“Many people currently use Citi Bike for a short trip or to solve a long walk, but the pedal-assist bikes will open up a completely new way of thinking about Citi Bike,” Julie Wood, a Citi Bike spokesperson, told Streetsblog.

Data released by Citi Bike shows that riders using pedal-assist bikes are using them for longer trips—including more trips over East River bridges—and that the bikes are getting more use in inclement weather. The e-bikes reach a top speed of 18 miles per hour, and the pedal assist feature—which kicks in as soon as you start pedaling—makes them easier on inclines and other challenging rides.

In addition to the e-bike expansion, Citi Bike also announced that it’ll add 13 new stations in “our busiest areas,” along with 2,500 new docks. Additionally, the company says it will “provide free upgrades after April 27 for members who dock electric bikes at certain stations along the L train corridor (more on that soon) or if there are only electric bikes at a Citi Bike station.”