The Prime Minister was joined by the Chancellor at a visit for the first time in several years in an effort to push the government's economic message.
The pair claimed more than 150,000 jobs could be created by £36 billion worth of infrastructure projects being started in the UK this year.
But they were forced to defend their record on tax - saying the scrapped 50p top rate sent a "terrible signal" and that they chose to raise the personal allowance over other tax changes.
During a question-and-answer session at the offices of leading construction company Skanska in Hertfordshire, they were asked why they did not adjust the threshold for the 40p rate in line with inflation to stop more middle earners being dragged into paying it.
Mr Cameron replied: "It was a French prime minister who once said to govern is to choose. We had to make a choice, how should we help people. And the choice we made was to help the lowest paid by taking over two million of them out of income tax altogether."