Convoys of go-karts evoking the hit Nintendo Co. Mario Kart series have become a head-turning sight among Tokyo's traffic, but the rental companies behind the vehicles, with a customer base comprised mostly of foreign tourists, face growing scrutiny over safety concerns.

Rental operators let customers drive often-crowded public roads in the low-slung vehicles while dressed as characters including those from the popular racing video games from the Mario franchise.

Despite the legality of the operations, the Japanese government has raised concerns over the lack of safety measures being taken by five Tokyo-based operators following a spike in the number of go-kart accidents in recent months.

On a cool evening in mid-June, this reporter witnessed a group of four Australians in their 20s riding go-karts. A man in the group said, "My wife checked it out, and we have seen various YouTube videos. They looked fun."

On their tour, two of the vehicles got into minor collisions near a crossing in Roppongi, the famous nighttime hub and one of the most heavily congested areas in central Tokyo.

The two karts, one that crashed into and slid on top of the other, disrupted traffic at the center of the three-lane road for a few minutes before the karts were disengaged by the two female drivers.

In Odaiba, also a popular sightseeing destination among foreign tourists, some of the vehicles from a group of six go-karts ran a red light so they could catch up with the cars ahead, prompting a warning from the tour guide to follow traffic rules.

In late May, the Metropolitan Police Department urged the five operators to beef up their safety measures.

The request, which is by no means binding, includes providing customers with a thorough explanation of local traffic rules and encouraging drivers to wear helmets and other protective gear.

Police also ask operators to check drivers' licenses; get customers to obey traffic lights; demand that they not drive erratically or get out of their vehicles to take photos when stopped at traffic lights; and ensure proper parking.

Perhaps the biggest concern is visibility, since go-karts hug the ground. Trucks, in particular, are being warned to keep a safe distance behind so as not to inadvertently drag the karts when making turns at intersections.

One of the five companies, EcoKart L.L.C., located in Higashiazabu in Minato Ward, did not appear overly concerned about implementing safety measures, even after receiving a written request from police in coordination with the transport ministry.

Although a man with the Australian group had failed to get an international driver's license before visiting Japan, one of the employees told him he would still be permitted to rent a go-kart as long as he assumed the responsibility, if stopped by police. The man decided against renting the vehicle.

An explanation of basic instructions prior to a drive takes less than 10 minutes. Drivers are shown how to adjust their seats, start and stop the go-karts, accelerate, brake, signal with indicators and use their parking brakes.

The guide, who was the lead driver among the six vehicles, appeared to try to provoke drivers to imitate his own maneuvers when he drove erratically in Odaiba, and asked drivers to park on the street for a 10-minute break during the two-hour ride.

The rental companies are believed to be taking advantage of traffic loopholes. Because karts are subject to conflicting traffic and safety laws -- falling into a legal grey area between cars and scooters -- drivers can avoid wearing helmets or seatbelts while using the vehicles, which reach speeds up to 60 kilometers per hour.

EcoKart's guide describes it as a "kind of an affiliate or franchise" of Shinagawa Kart L.L.P., another of the five service companies the police have asked to take further safety measures.

Shinagawa Kart manages over 100 go-karts for rental compared to only a dozen by EcoKart, according to the guide. But the Shinagawa outlet, which says in its guide it does not hire Japanese employees, accepts only foreign customers. Japanese customers are requested to make bookings at the shop in Higashiazabu.

Yusuke Yamazaki, the de facto head of the Shinagawa shop, and MariCar Inc., for which Yamazaki is the president, were sued at a lower court in Tokyo in late February by Nintendo over the alleged use of characters created by the Kyoto-based video game and console maker and acts of unfair competition.

Nintendo had received a complaint from residents over traffic violations around areas the go-karts are rented.

Yamazaki also works for several other operators in some major inbound tourist destinations in Tokyo, Osaka, the town of Fujikawaguchiko at the foot of Mt. Fuji, as well as Okinawa, according to their registries and court documents.

Costumed tourists riding go-karts in Tokyo have become a common sight, with many videos uploaded on the internet.

The go-kart service group, which counts foreign tourists as around 95 percent of its total customers, has estimated annual sales of at least 250 million yen ($2.2 million), which has grown since the establishment of MariCar in June 2015, a court document showed.

The defendants have argued MariCar is a separate entity that merely sells go-karts and provides maintenance services to the operators. The registries, however, show MariCar and the operators to be closely linked.

MariCar's Yamazaki had not responded to an interview request by Kyodo News by the time of this printing.

Amid rising safety concerns over the go-karts on public roads, Land, Infrastructure, Transport and Tourism Minister Keiichi Ishii indicated in late May that four fatal microcar accidents occurred on public roads in Japan between 2012 and 2016.

In 2016, 88 traffic accidents occurred in which drivers of microcars such as go-karts were to blame, according to the ministry.

Ishii also said traffic experts were scheduled to begin discussing safety measures for microcars in June, apparently referring to the go-karts popular among foreign tourists.

The Institute for Traffic Accident Research and Data Analysis, the organization Ishii mentioned at a press conference, however, had yet to conduct any research or analysis on the matter as of late June, its spokeswoman said.

Asked about further legal steps, a spokesman at the Metropolitan Police Department said, "Nothing has been decided on any action to take against the operators at present."

Payable insurance for go-kart drivers, if injured, is 3 million yen (about $26,300), with a 50,000 yen insurance deductible -- money paid out of pocket before the insurance kicks in.

While the processions of karts driven by costumed tourists are a welcome spectacle for many locals, who wave or take photos, safety concerns about their operation appear to have been largely brushed under the rug.