The convenience of the car versus the worthiness of walking? It's a difficult decision for some but a new service just launched in the US will help users compare the number of calories you'll use walking versus the amount of carbon dioxide emitted by driving.

The data will be provided in an update to the Navitime service, a route-mapping system that's accessible from PCs or four models of Windows Mobile 6.0 smartphones - the HTC Touch Cruise and Trinity, the Motorola Moto Q and the Samsung Blackjack II. A Java version is planned for later this month.

For example, suppose someone is at Yankee Stadium in New York and wanted to get to the Empire State Building. The Navitime software will give a couple of options.

Route 1 has you hopping in your car to drive the 15km. It will take an estimated 18 minutes and result in about 3.5kg of CO2 emissions.

Route 2 has you first walking a third of a mile -- and burning 26 kcal (109kJ)in the process -- to the 161 St. Yankee Stadium subway stop then jumping on the train line for a 23-minute journey to the 34 St. Herald Square station. Then it's a 0.3km walk - and 20 kcal (84kJ) burn - to the Empire State Building for a total journey time of 37 minutes.

Navitime is based in Tokyo and operates a popular service that's accessible through most cell phones in Japan. It recently began its expansion into the US market and the service is available at no cost until the end of September. The PC version and smartphone application can be accessed from www.navitime.com.

CO2 emissions aren't currently included for the rail journey because Navitime doesn't have enough information on U.S. subway transport to provide the estimates. Subway and rail cars are generally heavier and more polluting than those in Japan so the local calculations cannot be applied, Navitime said. However it plans to add CO2 emissions data for rail journeys soon.