Alphabet (GOOGL.O), self-driving unit Waymo is recalling nearly 3,900 robotaxis in the US because a software issue could cause the ​vehicles to enter a closed freeway construction zone and continue driving.

It ‌is the second recall by Waymo in just over a month.

The latest recall, filed with the National Highway Traffic Safety Administration (NHTSA), stems from more than a dozen incidents since early April in California and ​Arizona in which Waymo autonomous vehicles (AVs) did not recognise and drove past ramp ​closure signs into pre-planned freeway construction zones and freeway lanes with active ⁠construction work.

The company initially imposed freeway driving restrictions until it adopted improved awareness ​and responses to freeway closures. Waymo updated vehicle software to avoid entering construction zones.

Waymo ​said in a statement to Reuters on Thursday that it had “identified an area of improvement regarding performance around freeway construction zones. We voluntarily restricted freeway operations last month while making improvements.”

Last month, Waymo recalled ​about 3,800 robotaxis because vehicles could enter flooded roads with higher speed limits.

The ​recall followed an incident on April 20 in which a Waymo vehicle drove into a flooded ‌lane in ⁠San Antonio during extreme weather. Waymo said the vehicle was unoccupied and there were no injuries, but the incident prompted the company to review similar scenarios involving high speeds and impassable flooded roads.

Waymo has issued several recalls over the last two years, ​including over potentially ​inaccurately predicting the movement ⁠of a towed vehicle and over a vehicle’s detection response to pole or pole-like permanent objects.

Separately, Waymo is facing an NHTSA investigation after ​one of its self-driving vehicles struck a child near an ​elementary school ⁠in Santa Monica, California, in January, causing minor injuries.

In March, the National Transportation Safety Board said it was investigating an incident in January in which Waymo self-driving vehicles illegally ⁠passed a ​stopped school bus with its lights activated.

The issue ​of illegally passing stopped school buses had previously prompted a recall by Waymo last December.

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