Both the S5 and IS have child safety locks to prevent children from opening the rear doors. The S5 has power child safety locks, allowing the driver to activate and deactivate them from the driver's seat and to know when they're engaged. The IS’ child locks have to be individually engaged at each rear door with a manual switch. The driver can’t know the status of the locks without opening the doors and checking them.
To provide maximum traction and stability on all roads, All-Wheel Drive is standard on the S5. But it costs extra on the IS.
Earlier warning of stopped traffic, traffic signals, dangerous road conditions, weather, or accidents, can keep driver's safer and prevent crashes. The S5 has Car-to-X Services, a system that seamlessly communicates important warnings to the driver about impending danger, if they're available. The IS doesn’t offer a system that can receive automated systems from infrastructure.
To help make backing out of a parking space safer, the S5 has standard Rear Cross-Traffic Assist with Automatic Brake Activation, systems which detect vehicles approaching from the sides and can automatically apply the brakes to prevent a collision. Rear Cross-Traffic Braking costs extra on the IS.
The S5’s driver alert monitor detects an inattentive driver then sounds a warning and suggests a break. According to the NHTSA, drivers who fall asleep cause about 100,000 crashes and 1500 deaths a year. The IS doesn’t offer a driver alert monitor.
Both the S5 and the IS have standard driver and passenger frontal airbags, front and rear side-impact airbags, driver and front passenger knee airbags, side-impact head airbags, front seatbelt pretensioners, height adjustable front shoulder belts, four-wheel antilock brakes, traction control, electronic stability systems to prevent skidding, crash mitigating brakes, post-collision automatic braking systems, daytime running lights, lane departure warning systems, blind spot warning systems, rearview cameras, rear cross-path warning and available around view monitors.

