One has to assume that the noise source(the signal generated by the second car) will not match with the mother signal in duration. Therefore, it is better to observe the readings for a relatively large number of samples and the final decision should be over the greater length of time. The energy of the radar signal should be high to improve the SNR and it will enable to set the threshold of triggering SNR at a higher, more safe value.

We have tried to use ultrasonic sensors for reverse parking assistance but if another ultrasonic sensor is there near bye we are facing problem in estimating the correct distance.

I wish to know what is the solution for the problem we are facing.

I assume you are using a time of flight approach to measure distance.

To avoid interference you need for competing systems to use a multiple access technique in some way. This is very difficult in a situation where the competing systems are unplanned, there is no co-operation between them. You can use time division multiple access, TDMA(each user has a different time slot) and frequency division multiple access, FDMA (each user has a different frequency). However, for the standard car systems TDMA is complicated by the time of flight measurement and FDMA is difficult because the ultrasonic transmitters are highly resonant, they only really work at one frequency. I think the best solution would be CDMA, where the transmission is modulated with a code that is then matched filtered at the receiver. All systems have problems if their is no central planning though, just like all military RADARs can be jammed.