自己做的判定!大虾们就不用看了,只供新手参考!先看看效果:

说明：按键盘的向上方向键然后按向左或向右键可以观看效果

代码如下：

//定义速度变量
var speed:Number = 0;
var car_speed:Number = 0;
//增加键盘侦听器
Key.addListener(_root.road_mc);
road_mc.onEnterFrame = function() {
//键盘UP键按下
if (Key.isDown(38)) {
  if (road_mc._y<-80) {
   road_mc._y += speed;
   speed_func();
   car_speed_func();
   //trace(speed);
   car_mc._y>=166 ? car_mc._y -= car_speed : 正常;
  } else {
   road_mc._y = -437;
  }
  //键盘UP键放开
} else if (road_mc._y<-80) {
  if (speed>=0.25) {
   speed -= 0.25;
   road_mc._y += speed;
   //trace(speed)
   //trace(road_mc._y)
  }
} else {
  road_mc._y = -437;
}
//键盘LEFT键
if (Key.isDown(37)) {
  if (car_mc._x>76) {
   speed<3 ? car_speed=speed : car_speed=3;
   car_mc._x -= car_speed;
  }
}
//键盘RIGHT键    
if (Key.isDown(39)) {
  if (car_mc._x<378) {
   speed<3 ? car_speed=speed : car_speed=3;
   car_mc._x += car_speed;
   //trace(car_speed);
  }
}
//键盘DOWN键    
if (Key.isDown(40)) {
  if (speed>0) {
   car_mc<331 ? car_mc._y += 5 : 正常;
   speed -= 2;
  } else {
   speed = 0;
  }
}
//加速"shift"键  
if (Key.isDown(Key.SHIFT) && !this["press"]) {
  this["press"] = true;
  car_mc.play();
  speed<150 ? speed += 50 : 正常;
  var This = this;
  this["interval"] = setTimeout(function () {
   car_mc.gotoAndStop(1), This["press"]=false;
  }, 5000);
  //设置延时5000毫秒
}
};
//声明函数,按当前速度渐加
function speed_func() {
if (speed<10) {
  speed += 0.25;
} else if (speed<20) {
  speed += 0.5;
} else if (speed<30) {
  speed += 1;
} else if (speed<50) {
  speed += 1.5;
} else if (speed<80) {
  speed += 3;
} else if (speed<0) {
  speed = 0;
}
}

源文件下载：点击这里下载源文件