The primary responsibility for assisted driving still rests with the driver.

During the Golden Week holiday, more and more drivers hit the road with assisted driving features activated, but potential hazards have followed. Recently, on the Anshun section of the Shanghai-Kunming Expressway, a driver was using Navigate on Autopilot (NOA) while traveling on the highway, but when approaching within roughly 10 meters of a large truck ahead, the NOA suddenly disengaged and handed control back to the driver. At that moment, the vehicle was traveling at 120 km/h. The driver exclaimed: "With only about 10 meters between me and the rear of the truck ahead, there was simply no time to brake." In the end, the car rear-ended the truck directly. This incident has once again ignited discussion about the handover window of assisted driving systems.

One viewpoint holds that the primary responsible party for L2-level assisted driving remains the driver, who must stay alert while driving. However, another viewpoint argues that the driver was "thrown under the bus" by the assisted driving system, with the purpose being to avoid liability by ensuring the vehicle was not in intelligent driving mode at the moment of rear-end collision, thereby avoiding disputes. Some netizens commented: "At the very least, the assisted driving system should only disengage after confirming that manual takeover has occurred, or it should complete a safe pull-over procedure."

In reality, cases where assisted driving disengages at a distance of less than 100 meters from the vehicle ahead, or where the braking time after disengagement is less than 10 seconds, occur frequently. Last year, a vehicle on the highway prompted the driver about an obstacle ahead and transferred control to the driver in about 1 second, but a collision occurred less than 10 seconds after the handover. In recent years, more than 10 similar incidents have occurred, drawing widespread public attention.

Zhu Xichan, a former professor at Tongji University, previously stated in a media interview that achieving a 10-second TTC (Time to Collision) in highway scenarios is very difficult, as it requires the intelligent driving system to detect obstacles and notify the driver 10 seconds in advance, placing high demands on the vehicle's perception capabilities. When vehicle speed exceeds 100 km/h, the intelligent driving system needs to identify obstacles 300 to 400 meters ahead to meet the requirement. Zhu Xichan's team found through simulator testing that the average time from system alert to accident occurrence is only 1.7 seconds, while the average human driver needs 2.3 seconds to complete the process from recognizing the alert to effectively taking over. He said: "This is like asking an athlete to complete a sprint start 0.5 seconds before the starting gun fires."

At Li Auto's Livis Day software and embodied intelligence launch event, Li Auto CEO Li Xiang stated that an intelligent driving system instantly disengaging and handing control to a human is completely compliant from a regulatory standpoint and fully meets functional safety requirements, but for humans, it is precisely the most unsafe scenario. Nearly half of all intelligent driving accidents occur at the moment of handover, with drivers completely unprepared to face imminent danger.

The Key Laboratory of Road and Traffic Engineering of the Ministry of Education at Tongji University noted in "The Impact of Emergency Degree of Working Conditions on Driver Collision Avoidance Behavior" that rear-end accidents account for a very high proportion of total accidents, rising to 49% and 67% in highway and tunnel scenarios respectively. There are two main causes of such accidents: first, the following distance is too close for the driver to react in time; second, the driver's braking force is insufficient during collision avoidance. The laboratory results mentioned that when the initial headway is approximately 1.5 seconds, the driver's perception-reaction time is about 1.2 seconds, while when the initial headway increases to more than 2.5 seconds, the driver's perception-reaction time becomes very large, even reaching 3 seconds.

Based on factors such as driver perception-reaction time and causes of rear-end accidents, relevant regulations also set requirements for following distance. According to Article 80 of the Implementation Regulations of the Road Traffic Safety Law: "When a motor vehicle travels on a highway at a speed exceeding 100 kilometers per hour, it shall maintain a distance of more than 100 meters from the vehicle ahead in the same lane. When the speed is below 100 kilometers per hour, the distance from the vehicle ahead in the same lane may be appropriately shortened, but the minimum distance shall not be less than 50 meters."

Zhang Ming, a traffic safety technical engineer from the Sixth Brigade of the Highway Traffic Police Detachment of the Ningbo Public Security Bureau, mentioned in a 2023 paper titled "A Brief Analysis of Traffic Rules for Safe Driving Distance and Suggestions for Improvement" the "relationship between different speeds and braking distances for small ordinary passenger vehicles." If a small ordinary passenger vehicle is traveling at 110 km/h, the reaction distance is approximately 46 meters, and the braking distance (on dry road surface) is approximately 67 meters. Adding the two together, the vehicle's stopping and braking distance reaches 113 meters.

A smart connected vehicle R&D professional from an automaker told reporters that a remaining distance of just 10 meters is simply insufficient to complete braking. Moreover, currently only high-beam lidar on the market can detect distances of 300 meters, and in extreme weather or at night, the effective detection distance will be further shortened to about 150 meters. For ordinary passenger vehicles, achieving such detection range is difficult and comes with a high price tag.