Driver head locations: Considerations for head restraint design

<b>Objective:</b> U.S. FMVSS 202a requires that a vehicle head restraint lie within a specified distance (55 mm) from the physical headform on the head restraint measurement device (HRMD). Smaller values of this distance, known as backset, are frequently associated with improved protection against neck injury in rear impact. In some vehicles, small backsets are also associated with complaints of head restraint interference with drivers’ preferred head positions. The objective of this study is to examine head/head restraint distances using data from a lab study of driving posture to provide guidance for safe and comfortable head restraint design. <b>Methods:</b> Head positions were measured for 88 U.S. drivers in a laboratory mockup using a seat from a mid-size sedan. The head restraint was removed to allow measurement of drivers’ preferred head locations without interference from the head restraint. Rates of disaccommodation, defined as interference between predicted possible head restraint locations and drivers’ preferred head locations, were analyzed at HRMD-referenced backsets of 25, 50, 75, and 100 mm measured at 22° and 25° seat back angles. <b>Results:</b> With HRMD-referenced backsets of 25 mm and 50 mm measured at 25°, the head restraint intersected the preferred head locations of 17.9 and 5.2% of the drivers, respectively. An HRMD-referenced backset measured at 22° produced larger accommodation rates than the same backset measured at 25°. <b>Conclusions:</b> The reported distribution of occupant head positions and the resulting restrictions on comfortable head restraint position at various HRMD-referenced backsets and seat back angles help provide guidance for head restraint design. Knowing the actual mean driver-selected seat back angle for a particular vehicle seat and the model presented in this work, a manufacturer can choose a head restraint location that will have a high likelihood of complying with FMVSS backset requirements while also achieving minimal disaccommodation. The findings in this study support the flexibility in the current FMVSS 202a that permits testing at more upright seat back angles than the 25° originally proposed.

**研究目标：**
美国联邦机动车安全标准（Federal Motor Vehicle Safety Standards, FMVSS）202a规定，车辆头枕（head restraint）需与头枕测量装置（Head Restraint Measurement Device, HRMD）上的实体头模保持指定距离（55毫米）以内。该距离越小（即后移量（backset）越小），往往在后碰事故中对颈部损伤的防护效果越好。但在部分车型中，较小的后移量也会引发驾驶员抱怨头枕与自身偏好的头部位置产生干涉。本研究旨在通过驾驶姿态实验室测试获得的数据，分析头部/头枕间距，为安全且舒适的头枕设计提供参考依据。

**研究方法：**
本研究在实验室搭建的中型轿车（mid-size sedan）座椅样机中，对88名美国驾驶员的头部位置进行了测量。测试时移除头枕，以避免头枕干涉，确保测得的是驾驶员偏好的头部位置。本研究分析了在座椅靠背角（seat back angle）为22°和25°时，头枕测量装置参考下后移量分别为25、50、75、100毫米的场景下的不适配率（disaccommodation）——不适配率指的是理论可实现的头枕位置与驾驶员偏好头部位置之间的干涉比例。

**研究结果：**
当座椅靠背角为25°、头枕测量装置参考下的后移量为25毫米和50毫米时，头枕分别会与17.9%和5.2%的驾驶员的偏好头部位置产生干涉。在座椅靠背角为22°时测得的头枕测量装置参考后移量，其适配率高于相同后移量在25°靠背角下的适配率。

**研究结论：**
本研究测得的乘员头部位置分布，以及不同头枕测量装置参考后移量和座椅靠背角下舒适头枕位置所受的限制，可为头枕设计提供参考依据。汽车制造商若掌握某款车型座椅的驾驶员平均选择的座椅靠背角，并结合本研究提出的模型，即可选择既能大概率符合FMVSS规定的后移量要求，又能将不适配情况降至最低的头枕安装位置。本研究结果也证实了现行FMVSS 202a标准的灵活性——该标准允许采用比最初提议的25°更直立的座椅靠背角进行测试。