Professor Alan Hatcher, of the private International School for Security and Explosives Education, said changing car parking arrangements and traffic direction through the area could reduce the risk.
He argued that anti-terror measures could have unforeseen consequences, potentially increasing the number of casualties in an incident.
Prof Hatcher was giving evidence to the Commons select committee inquiry on transport security, which also heard police and aviation industry representatives clash on whether taxpayers or passengers should foot the bill for extra security measures.
"The movement of vehicles around Heathrow is phenomenal and, if we could define a better transport plan, this may alleviate some direct threats," he said. "Terminal three is currently a very easy target for a simple terrorist attack ... even a small device would result in large scale loss of life.
"As a result of our well-meaning security practices, we now see hundreds of people in lines that coil around each other. A well placed suitcase containing several kilograms of explosive left in the line would result in catastrophic fatalities and injuries."
Prof Hatcher urged the authorities to rely less on high-tech responses and focus instead on support for frontline staff. "In the main, these people are paid very poorly, they do not always have a structured career path and this can lead to a very high turnover of staff," he said.
Mike Todd, who deals with transport security issues for the Association of Chief Police Officers, called for a policing levy to be added to the cost of air tickets.
"I don't think it's right that so many people near airports have to pay for policing and security. Fifty pence on each passenger would pay for the price of policing," Mr Todd, the chief constable of Greater Manchester police, said.
In his submission to the committee, he warned that the risk assessment process involving the police and the aviation industry "lacks dynamism, accountability and resilience".
He said it suffered from a lack of investment and had a "bias towards cost saving at the expense of security outcomes".
However, British Airways said the regime was too strict and that the government should pay for the extra safety measures needed on British flights.
"BA believes there is over-regulation in the UK when compared with security regimes existing in both Europe and the US in particular," the airline said in its submission. "This compromises the competitive position of the UK's airlines and airports."
However, it added that it believed Britain required the highest standards of security anywhere in the world, bar Israel.
"The higher level of threat faced by UK aviation can be attributed to government policy ... additional [safety] measures, imposed by individual countries as a result of an increased threat due to nationality, must be government funded," it said.
