Here is a video of Bootstrap in its basic configuration. It is driving around the inside of a 24″ box using the ultrasonic sensor and the bumper. As you can see the ultrasonic does a good job of avoiding collisions as long as it faces relatively directly on to the obstacle. At very sharp angles it cannot detect the obstacle (because the sound waves are not being bounced back into the detector), and the robot has to depend on the bumper to find the obstacle and choose another route. This problem could be solved a number of ways: putting the ultrasonic sensor on a servo so it can track left and right, using more than one fixed ultrasonic, or adding some side facing IR sensors to the single ultrasonic. Any of these will be an easy option to add to the basic Bootstrap configuration.