This paper presents the design and implementation of an intelligent transportation web service (ITWS) and studies various QoS requirements for the web service infrastructure. In particular, we pay attention to the performance issues on ITWS data transmission and compression. Since SOAP message is a textual document, we can improve the data transmission time by compressing data on transmission. In our experiment, compressing data before or after SOAP serialization both can reduce the size of transmitted message down to as little as 2% and cut the response time to a quarter of the original message.