Researchers have improved travel demand forecasting methods in recent decades but invested relatively little to understand their accuracy. A major barrier has been the lack of necessary data. We compiled the largest known database of traffic forecast accuracy, composed of forecast traffic, post-opening counts and project attributes for 1291 road projects in the United States and Europe. We compared measured versus forecast traffic and identified the factors associated with accuracy. We found measured traffic is on average 6% lower than forecast volumes, with a mean absolute deviation of 17% from the forecast. Higher volume roads, higher functional classes, shorter time spans, and the use of travel models all improved accuracy. Unemployment rates also affected accuracy—traffic would be 1% greater than forecast on average, rather than 6% lower, if we adjust for higher unemployment during the post-recession years (2008 to 2014). Forecast accuracy was not consistent over time: more recent forecasts were more accurate, and the mean deviation changed direction. Traffic on projects that opened from the 1980s through early 2000s was higher on average than forecast, while traffic on more recent projects was lower on average than forecast. This research provides insight into the degree of confidence that planners and policy makers can expect from traffic forecasts and suggests that we should view forecasts as a range of possible outcomes rather than a single expected outcome.

近几十年来，研究人员改进了旅行需求预测方法，但在了解其准确性方面投入相对较少。一个主要障碍是缺乏必要的数据。我们编制了已知最大的交通预测准确性数据库，由美国和欧洲 1291 个道路项目的预测交通、开通后计数和项目属性组成。我们比较了测量流量与预测流量，并确定了与准确性相关的因素。我们发现实测流量平均比预测流量低 6%，平均绝对偏差为 17%。更高容量的道路、更高的功能等级、更短的时间跨度以及旅行模型的使用都提高了准确性。失业率也影响准确性——交通量将比平均预测高 1%，而不是低 6%，如果我们在经济衰退后的年份（2008 年至 2014 年）调整较高的失业率。随着时间的推移，预测的准确性并不一致：最近的预测更准确，平均偏差改变了方向。从 1980 年代到 2000 年代初期开放的项目的平均流量高于预期，而最近项目的平均流量低于预期。这项研究深入了解了规划者和政策制定者对交通预测的信心程度，并建议我们应该将预测视为一系列可能的结果，而不是单一的预期结果。从 1980 年代到 2000 年代初期开放的项目的平均流量高于预期，而最近项目的平均流量低于预期。这项研究深入了解了规划者和政策制定者对交通预测的信心程度，并建议我们应该将预测视为一系列可能的结果，而不是单一的预期结果。从 1980 年代到 2000 年代初期开放的项目的平均流量高于预期，而最近项目的平均流量低于预期。这项研究深入了解了规划者和政策制定者对交通预测的信心程度，并建议我们应该将预测视为一系列可能的结果，而不是单一的预期结果。