Have a look at this paper:

http://robotics.eecs.berkeley.edu/~sastry/pubs/PDFs%20of%20Pubs2000-2005/Pdfs%20of%20Misc.Others/Sharp,Cory/SharpDesign.pdf

The following sensors are also use for counting of vehicles.

1)Piezoelectric Sensor

2)Acoustic detector

However,the following attached paper may also be helpful.

From your definition you need cameras and image processing algorithms. Normal sensors will not detect the type of the cars. However, if you only wants to count numbers only, motion detectors, distance sensors can be used.

Moreover, way you need a WSN? you can create only one sensor node with a GSM module to transfer the data to your central node or computer. IF you want to build this system try Arduino + GSM + ultrasonic sensors and motion detectors. It will cost less than 80$

Hi;

I advice you to used "Arduino + GSM + ultrasonic sensors " as Dr.Mohammed said, also for monitoring , you can update the sensed datato the cloud directly. For example ThingSpeak . I hope this link will be helpful http://community.thingspeak.com/tutorials/arduino/send-data-to-thingspeak-with-arduino/

*Note : you can take the following scenarios in consideration

1. IF  the same gate use for IN/OUT you will need one bidirectional sensor!

2. IF The city has two gates one for in and other for OUT you will need two sensors.

Instead of using WSN, this vehicle detection can be done using RFID tags along with using Ardunio boards.