C :: Write A Interrupt To Add Another Sensor For Robotic Car?

This is a program that is for a robotic car. The car has two light sensors on the front to detect when it veers off the road. When a sense is detected on one of the sensors the corresponding motor will halt until the sensor again detects the track. Again two sensors, left and right, and two motors (one per wheel). I have case statements to do the two left and right sensors.

how would I implement a third sensor with an interrupt to halt the car after it senses a stoping line. E.g. the sensor in the middle detects the stopping point so both motors halt.Here is the code I have so far...The cases are the sensor detections.

Code:

#include <hidef.h> /* common defines and macros */
#include "derivative.h" /* derivative-specific definitions */
// Function Prototypes
void init_PWM(void);
void initLCD(void);

[code]....
}