[exelius]

Government spending on capital projects generally causes an increase in economic activity. The government initiates economic activity, which causes companies to hire more workers, invest in new equipment, purchase supplies, etc. That all eventually makes its way into peoples' pockets, where they spend the money and throw it back into the economy. Capital projects tend to have an outsized impact because much of the economic activity required to build them wouldn't be practical to source from outside the US (you would never buy concrete from China, for example).

Furthermore, assuming the capital projects filled a gap in the economy, they may make it easier to do business such that companies can operate with reduced costs relative to global competition. That would be a secondary effect from having improved infrastructure, but it would be much longer-lasting than the primary effects.