Recently, multitasking on smart phones during navigation has emerged as a problematic social behavior due to its potential danger. In this paper, we propose to alleviate the situation by employing a sensor system for obstacle detection and aid the multi-tasking user so that one can safely navigate and carry out the on-going secondary task as effectively as possible. As such, we have implemented an ultrasonic sensor system interfaced into the smart phone that can constantly appraise the user of the incoming obstacles. We ran experiments to validate our approach, checking whether such a system would help user bump less into obstacles than without, and observe their multitasking behavior such as the physical attentional switch. Our experiments have shown that with the aid of the sensor system, the user's attention switch was significantly reduced, however, there was no differences in the performance (e.g. no. of collision) because the momentary strategic spatial and path planning was quite effective with slow pace and light pedestrian traffic. We conclude therefore human dual or multitasking ability is sufficient to overlap "casual" video watching and "slow" navigation. However, with more stringent task, e.g. text messaging and heavier traffic, we expect the reduced attention shift to ultimately improve the performance as well.