hello Alex. thanks for remark, you are right. However my "ultimate" goal is to count people passing a back-country skiing track, so I decided to emphasise the human-counting use case.
There are some non-trivial design problems that need to be overcome in outdoor environments. The ultrasonic sensor may not be a good choice for winter counting, frost buildup on the sensor surface will likely greatly affect performance, plus it's a bit of a power hog.
Don't forget that battery capacity/max current can be greatly affected by cold temperatures as well.