Arduino speed detector

//code by YaSh.

int sen1=A0;
int sen2=A3;
int ledPin=9;
unsigned long t1=0;
unsigned long t2=0; 
float velocity;
float velocity_real;
float timeFirst;
float timeScnd;
float diff;
float speedConst=7.5;  //in cm.
void setup()
{
  Serial.begin(9600);
  pinMode(sen1,INPUT);
  pinMode(sen2,INPUT);
  analogWrite(11,LOW);
  analogWrite(10,HIGH);
}
  void loop()
  {
    if (analogRead(sen1)<500 && analogRead(sen2)>500)
    {
      timeFirst = millis();
      digitalWrite(ledPin, LOW);
      delay(30);
    }
 
  if (analogRead(sen2)>500 && analogRead(sen1)<500)
  {
    timeScnd = millis();
    diff = timeScnd - timeFirst; 
    velocity = speedConst / diff;
    velocity_real = (velocity*360)/100;     //milliseconds to hours and centimetres to kilometeres.
    delay(30);
    digitalWrite(ledPin, HIGH);
  Serial.print("\n the velocity is : ");
  Serial.println(velocity_real);
  Serial.print(" km/hr. ");
  delay(500);
  digitalWrite(ledPin,LOW);
  delay(500);
  }
  /*else if(analogRead(sen2)<500 && analogRead(sen1)<500)       //uncomment if you want to write it.
  {
      Serial.print("\n Error:404/ the object is tooo big.");
  }*/
 /* else{Serial.print("\n error:404/no object detected ");}     //uncomment if you want to write it.*/
  }