As Memorial Day kicks off the start of the summer travel season, Americans could see some of the best travel offers in years. A combination of lower airfares, cheaper gas prices, a strong dollar and recent world events is responsible for increased availability and high savings, according to Travelzoo. Compared to last year, the travel deal website predicts Americans could expect to save up to 40 percent on summer travel.

Here are five trends driving down summer travel prices:

1. Lower Airfares

Travelers are seeing the lowest flight prices in years. Prices have declined 12 percent from 2015, and over 20 percent from two years ago. Lower oil prices have decreased airline-operating costs, while major carriers have dropped fares to compete with budget carriers that are adding more routes.

2. Dollar Impact Abroad

The strong dollar means Americans can spend less to travel internationally. According to the National Trade and Tourism Office, U.S. outbound travel in January rose 9 percent year over year. In the same month, travel to Canada increased 19 percent year over year. With the Canadian dollar trading at U.S. 76 cents, Americans receive a 24 percent discount on all costs.

3. Deals Across Europe

The recent wave of terror attacks in Paris, Brussels and Turkey has impacted tourism in the region. Hotel consultancy MKG Hospitality reported that Paris occupancy rates in April were down 11 percent year over year, with 30 percent of rooms in the city left vacant.

As a result, hotels and tourism boards are trying to entice tourists with lower rates and other incentives. In France, the state railway operator is offering discounted train tickets, while hotels are offering discounts of up to 30 percent. In Turkey, prices are down by more than 40 percent, with five-star hotel rooms available for less than $50.

4. High Demand for Asia

Lower airfares are translating into savings of more than 35 percent for trips to Asia and Australia. For example, airfare to China is at some of the lowest rates in years. Advertised on its website, Travelzoo cites an eight-night escorted vacation to China, including breakfast and air, for under $850.

5. Tropical Island Availability

Travelers are rethinking their plans to visit the Caribbean and Mexico in the wake of the Zika virus. A February Travelzoo poll found that 30 percent of travelers were changing their plans to avoid Zika-impacted destinations. Many hotels, resorts, vacation packages and cruise lines are offering steep discounts in an effort to overcome the Zika stigma.