You may feel a sense of dread on the road these days, watching other drivers gab on phones and fiddle with iPods. But by one measure, American motorists are safer than they've been in decades -- though it's not clear exactly why.

The rate of fatalities per 100 million miles of vehicle travel on U.S. roads is at its lowest since the government began keeping such data in the mid-1970s, according to the National Highway Transportation Safety Administration. The NHTSA's recent projected decline in overall traffic deaths for the first half of 2009 was played as a 'silver lining' to the lousy economy: Fewer people driving to work, fewer people dying on the road.

That could be true. The raw number of highway deaths tends to drop during recessions, according to government figures.

But more significant is the decline in the death rate, or deaths spread across miles of driving -- long division that helps smooth out effects of economic cycles. Based on the latest estimates, there were 1.19 deaths for every 100 million vehicle miles traveled in the second quarter of 2009. That compares with an estimated rate of 1.1 for the first quarter.

By comparison, the deaths-over-miles-traveled ratio was 1.46 for all of 2005 -- a year in which 43,510 people died on the roads. If current trends continue, it's possible that fewer than 35,000 people will die on U.S. highways this year.

'We are hitting fatality rate levels we couldn't have imagined a decade ago,' says NHTSA spokesman Rae Tyson.

Exactly why driving is less deadly isn't clear, though the trend has been going on for several years. The data don't explain the cause of every accident nor why accidents that could have been deadly weren't.

One factor may be better safety technology. A decade ago, 77% of vehicles sold had no side airbags, according to Insurance Institute for Highway Safety data. In 2009, only 10.3% of cars didn't offer side airbags -- and nearly 65% did offer head- and torso-protecting airbags as standard equipment.

Electronic stability control -- systems that use the car's brakes to prevent skids -- were available on just 9% of vehicles sold in the U.S. in 1999. For 2009, 74% of cars and 100% of sport-utility vehicles sold offered the technology as standard equipment, and 85% of vehicles sold overall offered it as either a standard or optional feature.

Highways also have more safety features. Rumble strips, which alert drowsy drivers when they are drifting off the pavement, are more common, as are barriers that prevent vehicles from sliding across medians of divided highways.

And believe it or not, drivers are behaving better, too. More U.S. motorists than ever are wearing seat belts -- 84% by the most recent government estimates, up from about 60% in the mid-1990s. 'Click It or Ticket' campaigns annoy some people, but they also apparently have motivated many seatbelt-averse people to change their ways. Tougher speed enforcement and campaigns against drinking and driving could be holding the fatality rate down as well.

如今开车行驶在路上，看到其他司机对着手机喋喋不休或者是摆弄手里的iPod,你或许会感到一丝不安。不过从某一个方面来看，美国司机的驾驶安全系数却是数十年来最高的──尽管个中缘由还不是很清楚。

美国国家高速公路安全管理局（National Highway Transportation Safety Administration）的数据表明，自上世纪70年代中期有数据记录以来，美国每1亿英里驾驶里程的事故死亡人数目前处于最低的水平上。高速公路安全管理局最近发布了2009年上半年交通事故死亡总数预计将出现下降的消息，这不啻于经济阴霾下的"一丝光亮":开车上班的人少了，因此交通事故丧生的人也少了。

这种说法可能没错。美国政府的数据表示，高速公路死亡人数在经济衰退期间有下降的趋势。

不过，死亡率的下降，也就是按英里数计算的死亡人数的下降更为重要，因为这种计算方法有助于消除经济周期的影响。根据最新的预测数据，在2009年第二季度，每1亿英里驾驶里程的死亡人数为1.19人，而在第一季度，这个数字为1.1人。

与之相比，2005年全年的英里驾驶里程死亡人数为1.46人──当年共有43,510人死于车祸。如果目前的这种趋势能够继续保持，那么今年高速公路死亡人数有望少于3.5万人。

高速公路安全管理局的发言人泰森（Rae Tyson）表示，我们正在接近的这一交通事故死亡比率在十年前是无法想象的。

至于驾驶安全为什么会提高，没有人能够说得清，尽管这一趋势已经持续了数年之久。从美国政府的这一数据没有提供交通事故的原因，以及为什么从前可能致死的事故如今却没有那么可怕。

一个原因或许是更先进的安全科技。美国高速公路安全保险协会（Insurance Institute for Highway Safety）的数据显示，十年前，在美国销售的车辆中有77%没有安装侧气囊。而在2009年，只有10.3%的汽车没有配置侧气囊；而近乎65%的汽车将头部气囊和躯体固定气囊列为标准配置。

安装了电子稳定控制系统的汽车在1999年时仅占在美销售车辆的9%.而在2009年，在美销售的74%的轿车和全部运动型多功能轿车（SUV）将这一技术作为标准配置；而在美销售的所有车辆中，有85%将电子稳定控制系统作为标准配置或选择性配置。电子稳定控制系统能够利用汽车的刹车装置防止打滑。

高速公路也安装了更多的安全设施。振动带以及隔离带都更为普及。振动带能够在困倦的司机偏离车道时起到提醒的作用，而隔离带能够防止汽车越线开向逆行车道。

另外，不论你是否相信，司机们的表现也更好了。在美国，系安全带的司机人数达到了前所未有的水平──最新的政府估算数据显示，有84%的司机系上了安全带，这比上世纪90年代中期的60%增加了不少。在美国发起的"系安全带，还是吃罚单"运动惹恼了一些人，不过这也让许多不愿系安全带的人改变了他们的作法。更为严格的限速措施和针对酒后驾车发起的一系列整治措施也对车祸死亡人数起到了抑制作用。