The A5 has standard head airbag curtains for front and rear seats which act as a forgiving barrier between the driver and outboard passenger's upper bodies and the window and pillars. Combined with high-strength steel door beams and lower side airbags this system increases head protection in broadside collisions. The Z4 doesn't offer side airbag protection for the head.
The A5 has a standard Audi Backguard System, which use a specially designed headrest to protect the driver and front passenger from whiplash. During a rear-end collision, the Audi Backguard System moves the headrests forward to prevent neck and spine injuries. At the same time the pretensioning seatbelts fire, removing slack from the belts. The Z4 doesn’t offer a whiplash protection system.
The A5 has all-wheel drive to maximize traction under poor conditions, especially in ice and snow. The Z4 doesn’t offer all-wheel drive.
The A5’s standard lane departure warning system alerts a temporarily inattentive driver when the vehicle begins to leave its lane and gently nudges the vehicle back towards its lane. A lane departure warning system costs extra on the Z4.
The A5 Premium Plus/Prestige has a standard Top and Corner View Cameras to allow the driver to see objects all around the vehicle on a screen. The Z4 only offers a rear monitor and front and rear parking sensors that beep or flash a light. That doesn’t help with obstacles to the sides.
Both the A5 and Z4 offer rear cross-traffic warning, but the A5 with Rear Cross-Traffic Assist also has Automatic Brake Activation (automatically applies the brakes) to better prevent a collision when backing near traffic. The Z4’s Cross Traffic Warning doesn’t automatically brake.
Both the A5 and the Z4 have standard driver and passenger frontal airbags, front side-impact airbags, driver and front passenger knee airbags, front seatbelt pretensioners, four-wheel antilock brakes, traction control, electronic stability systems to prevent skidding, crash mitigating brakes, post-collision automatic braking systems, daytime running lights, rearview cameras and available blind spot warning systems.