To improve their capability to estimate the emissions reduction benefits of new bicycle facilities, the Association of Monterey Bay Area Governments and the Monterey Bay Unified Air Pollution Control District in California sought to develop a new software tool. The tool was required not only to deliver accurate forecasts with a small development budget but also to operate independently of other models, run on a platform that could be freely distributed without special software licenses, and provide user-friendly access to planners in multiple agencies with varying levels of technical skill. For these requirements to be satisfied, an incremental nested logit mode choice model was developed that pivoted off static exports of trip tables from the Association of Monterey Bay Area Governments' existing four-step travel model. GPS traces collected from smartphone users in the region were analyzed to estimate a path-size logit route choice model, and the California Household Travel Survey was used to estimate a scaling coefficient on the best route utility. The model was implemented in an Adobe ActionScript graphical user interface. After the user edits the bicycle network in the user interface, new bicycle levels of service are skimmed, and new shares for each mode are calculated from their original shares in the no-build alternative and the change in the bicycle level of service. Finally, the emissions reduction is estimated on the basis of the distance and average speed of the vehicle trips replaced by bicycle travel. The result is an accurate, fast, freely distributable, user-friendly tool that is consistent with the forecasts produced by the four-step model.