Driving in Japan's Summer Heat and Rain: A Rental-Car Safety Plan

What should I do before leaving the rental-car counter?#

Summer driving in Japan is manageable when the plan is made before the first kilometre. July and August can bring high heat, humidity, sudden thunderstorms, and heavy rain. The risk is not only discomfort: glare, wet roads, tired passengers, a blocked route, or a flooded underpass can turn a short drive into an emergency.

Use this decision flow before setting off. One person should check it while the driver is parked, not while moving.

Check	Question	Decision
1	Is there a JMA warning or advisory on the route?	Delay, shorten, or reroute; do not treat a warning as a normal forecast
2	Does the road operator show a closure, restriction, or serious delay?	Follow the notice and choose an official alternative
3	Are passengers hydrated, rested, and able to tolerate the heat?	Take a break or change the plan before driving
4	Is every child and pet coming out at every stop?	Take them with you; never leave them in the parked car
5	Is the car showing a warning light or unusual smell?	Stop the trip and call the rental company

Save the rental company’s emergency number before leaving the counter. It is not a single nationwide rental-car number: use the number on your agreement, key tag, or handover sheet. Also note the car’s registration number and your destination in a message or paper note, so a passenger can report them without searching through luggage.

How do heat and sudden rain change the drive?#

Heat makes a car’s cabin, steering wheel, and child restraints hot after parking. Air conditioning helps while travelling, but it does not make an unattended parked vehicle safe. Never leave a child or pet alone in a parked car, even if the engine or air conditioning is running. A vehicle can stop, the cooling system can fail, or the car can move if it is not secured correctly. Take everyone into the building or bring them to a supervised, shaded place.

Before departure, set the route while stopped, carry water for the passengers, and schedule a real break rather than waiting for dizziness, headache, nausea, confusion, or unusual fatigue. If a driver develops those symptoms, stop in a safe place and get help; do not ask that person to “push through” to the next attraction.

Rain creates a different set of traps:

• The first rain can make a road slick with dust and oil. Brake gently and leave more space.
• Spray from buses and trucks can hide lane markings. Reduce speed and avoid abrupt lane changes.
• A bright wet road can hide standing water. Do not use cruise control in heavy rain.
• A sudden cloudburst can overwhelm drains and turn an underpass or dip into a flood before the water looks deep.

If visibility collapses, keep both hands available, slow smoothly, and look for a safe place to leave the traffic stream. Do not stop under a bridge, at a tunnel mouth, on a bend, or where another driver cannot see you.

How should I use road notices and weather warnings?#

Check the JMA forecast and warnings for the departure area, destination, and every mountain or coastal section between them. Then check the relevant expressway operator or the Japan Road Traffic Information Center through official road information. NEXCO road notices can show closures and restrictions; a route that was open at breakfast may not be suitable after a downpour.

Do not rely on a navigation app’s fastest route alone. Compare its route with road signs and official notices. If a sign says a road is closed, flooded, or restricted, obey the sign even if the navigation system still draws a line through it. Ask the rental desk which local roads are unsuitable for a small car, and keep a passenger responsible for checking updates while the driver concentrates on the road.

Common failure patterns include:

1. Leaving after seeing a warning but planning to “look at it later.” A delay at the start is safer than reaching a closed mountain road with no easy turn around.
2. Following the car ahead into water. The vehicle in front may be taller, may know the road, or may simply have made a bad decision. Its movement is not proof that your car can pass.
3. Stopping wherever the rain becomes frightening. A blind curve or motorway shoulder can create a second danger. Continue slowly to a safe stopping place if possible; use hazard lights only as a warning, not as permission to park anywhere.
4. Continuing after a warning light or water entry. A rental car is not yours to diagnose beside traffic. Call the company and follow its recovery instructions.

What should I do if the road is flooded?#

Turn around before the water. Do not drive into an underpass, a road hidden by runoff, or any section where you cannot see the surface, lane edge, drain, or depth. Even shallow-looking water can conceal a drop, debris, or a washed-out road. Moving water can push a vehicle sideways, and water can enter the engine or electrical system.

If the car becomes stuck in water, keep the doors and windows available for escape, call for help, and follow emergency instructions. Do not restart an engine after water has entered the vehicle. Move people to a safe, higher place only if you can do so without entering moving water. Call 119 for fire or an ambulance if someone is injured or trapped; call 110 for police. Report the exact road, nearby sign, direction of travel, and car registration to the rental company as soon as it is safe.

What is the safest response to a breakdown or heat problem?#

If the car loses power, overheats, or shows a serious warning light, move as far from traffic as you safely can. On an expressway, use a designated emergency phone or roadside assistance process where available, and do not walk along the carriageway. Keep passengers away from traffic and wait for the rental company’s instructions.

For a non-urgent problem, call the rental company’s emergency number first. Give:

Information to report	Example of useful detail
Location	Expressway name, direction, nearest interchange, service area, or kilometre marker
Vehicle	Registration number, rental agreement number, and car colour/type
Problem	Warning light, smoke, flat tyre, water entry, or loss of power
People	Number of passengers and any child, pet, injury, or heat illness
Immediate risk	Traffic position, darkness, floodwater, fire, or blocked exit

If there is immediate danger, call 110 or 119 before waiting for the rental company. JNTO’s 24-hour Japan Visitor Hotline can provide English support, but it does not replace emergency services. As of July 2026, the practical rule is simple: check the route before driving, stop before water, take every child and pet out of the car, and ask the rental company before moving a damaged vehicle.