A green light means traffic is moving forty-five miles an hour,yellow means thirty to forty-five miles an hour,and red means heavy traffic— cars standing still or moving less than thirty miles an hour.
“See that red light near Austin Avenue(大道)?”the controller asked a visitor.“There's a repair truck fixing the road,and the traffic has to go around it.”
At the Roosevelt Road entrance to the expressway,the light kept changing from green to red and back to green again.“A lot of trucks get on the expressway there,”the controller explained.“They can't speed up as fast as cars.”
The sensors(传感器) show immediately where an accident or a stopped car is blocking traffic,and a truck is sent by radio to clear the road.The system has lowered the number of accidents by 18 percent.There are now 1.4 deaths on Chicago's expressways for each one hundred million miles travelled,while in other parts of the country there are 2.6.
Traffic experts say that the Chicago system is the “coming thing”.Systems like Chicago's are already in use on some expressways in Los Angeles and Houston.“Chicago has taken the lead.”says New York City's traffic director;and he adds,“we are far behind…”

绿灯意味着车辆移动速度为每小时45英里，黄灯为30-45英里，红灯
则意味着交通繁忙-车辆停滞不动或移动速度低于每小时30英里。
“看见奥斯汀大道附近的那盏红灯了吗？”调度员问来访者。“那是因为有辆车坏在路上，其它车辆必须从它旁边绕行。”
在罗斯福路通往高速公路的入口处，灯光不断地从绿色变成红色，又从红色转回绿色。调度员解释说：“太多货车要上高速公路了，这些货车比其它车辆行驶地慢。”
当某处发生交通事故或有停驶车辆阻碍交通时，传感器马上就会显示情况，并用无线电指派车辆去疏通道路。这套系统可使事故数量降低18%。目前，在芝加哥高速公路上每1亿英里行驶距离的死亡人
数为1.4人；而美国其它地区为2.6人。
交通专家表示说：“芝加哥的系统是大有前途的东西。”类似芝加哥的这种系统已被用于休斯顿和洛杉基的一些高速路上。纽约市的交通主任说：“芝加哥带了个头，”接着他又说，“我们大大落后了。”