# Advances of UAVs toward Future Transportation: The State-of-the-Art, Challenges, and Opportunities
## 3 State of the Art of Practice and Research
### Challenges and Opportunities  
UAVs seem invaluable to the transportation industry in reducing time, cutting costs, increasing work quality, and even saving human lives. Topographic surveys for site visualization can be carried out ten times faster and with better precision by UAVs than the traditional manual surveys [98]. UAVs have also been reported to reduce infrastructure health assessment costs up to 90%, in some cases [99]. Search and rescue UAVs have played a crucial role in saving lives during a disaster, and various emergency scenarios, with more than 500 lives reportedly saved so far [100]. These are some of the many contributions of UAVs, and these statistics are only going to improve further with the advancement of UAV technology. However, the use of UAVs in the future transportation sector comes with certain challenges and limitations. A current technological shortcoming regarding UAVs for logistics and delivery is that the battery life of UAVs is limited, which constraints the operational range. Commercial UAV batteries currently offer a flight time of around 10 to 20 min, rarely up to a couple of hours. Flight time and battery life are also dependent on the UAV's weight as well as the weight of the shipment [101]. Due to the energy limitation, UAVs are often used for temporary or short time purposes. The long-term mission of UAVs remains a challenge. Although the technological advancement of lithium-ion batteries is increasing as the year progresses and the average battery life is predicted to double by 2025, it will still be some time before a fleet of UAVs can replace the ground transportation delivery trucks that can operate for a full day, and beyond. Thus, the current opportunity for UAVs is to complement the delivery truck to reduce operating costs and minimize the delivery time. In addition to the onboard battery capacity, similar to other autonomous ground vehicles, UAVs also suffer from a longer charging time compared to refueling time in traditional manned aircraft. However, there is no doubt that the flight time of commercial UAVs will improve without recharging, enabling them to deliver more goods along with the advancements in the energy sectors, such as renewable energy sharing, faster-charging process, and more extensive portable energy storage. For continuous operation of battery-based UAVs, swapping, laser-beam inflight recharging, and tethered technologies are being investigated [102]. Swapping enables  
Figure 8: Post-disaster mapping mission, and user and ground control station use case [68]. the UAVs to recharge their batteries using docking stations. Laser-beam inflight recharging technology includes the transfer of light energy to UAVs in-flight and the conversion of that energy to electricity. Tethering refers to the continuous power supply through power lines. A hybrid power supply system is also preferable for UAVs for increasing their endurance and operating times. The hybrid power system typically involves combining the UAV battery with various energy sources such as fuel cells, solar cells, and supercapacitors. Many researchers studied the trade-offs between shorter flight time and UAVs' operating and maintenance costs to overcome the previous issue and make UAV delivery more practical. More research is needed to address certain UAV aspects like localization and navigation, UAV coordination, and UAV designs towards implementing drones in the logistics sector [103]. UAV-enabled surveillance works much better with multiple UAVs rather than a single UAV. Due to this, efficient and accurate multi-UAV cooperation algorithms for more advanced data collection and sharing should be developed [103]. [PERSON] et al. identified UAV regulations challenges and threats to privacy and security as one of the most critical barriers to realizing the UAVs for logistics purposes [104]. In disaster scenarios, scouting the area with automated UAVs might not be apt as disaster situations require time-critical actions and UAVs have limited battery life.