The 911 attacks were designed to minimize casualties at the WTC and specifically targeted offices at the Pentagon.

WTC
The number of casualties on 911 was extremely low. 2,996 people died. This is an incredibly low number considering what could have been. If the Russians had planned their attack for noon, they would have maximized their causalities. 50,000 people worked at the WTC and 200,000 people visited daily. Out of that quarter of a million people only 2,606 people died at the WTC. That’s only a little over 1%. Out of the number of 50,000 workers, only 5% of the people were killed. Therefore the attackers aimed to decrease the provocation by keeping the casualties low.

How they did it. 
Beginning the attacks before the end of rush hour was essential. At 0846 hours the first plane hits the North Tower of the WTC. NYC’s rush hour runs from 7am to 10am. The morning rush was a little over halfway done and New Yorkers normally start their day later than most cities. Their median arrival time is 0824 hours (Silver 2018). So, 0846 hours was chosen in order to minimize the casualties. The attackers also decided to place their first hit on higher floors than the second hit. Flight 11 struck floors 93-99 of the North Tower leaving 89% of the floors beneath intact. Flight 175 struck lower down on the South Tower hitting floors 75-85 leaving 72% of the floors intact. The timing of the attacks also meant that people going to work decided not to go to their office.
The Pentagon
The Offices of the Defense Intelligence Agency and the Naval Operations Center (DIA & NOC) were targeted that day. A sure sign of Russian revenge for the sinking of the Kursk. The attacks on the DIA & NOC were intentional. The offices were located inside an inner ring of the first floor of the Pentagon. These were precision attacks and could only have been conducted by an autopilot. In the 1990’s Russian intelligence had penetrated Honeywell, the manufacturer of the avionics used in 757-767 airliners. They were in a position to hack the auto pilots. 

Sources:
History TV. (2018). History TV. [online] Available at: https://www.history.com/topics/world-trade-center [Accessed 18 Jun. 2018].
Plumer, B. (2013). Nine facts about terrorism in the United States since 9/11. The Washington Post. [online] Available at: https://www.washingtonpost.com/news/wonk/wp/2013/09/11/nine-facts-about-terrorism-in-the-united-states-since-911/?utm_term [Accessed 18 Jun. 2018].
Silver, N. (2018). Which Cities Sleep In, And Which Get To Work Early. [online] FiveThirtyEight. Available at: https://fivethirtyeight.com/features/which-cities-sleep-in-and-which-get-to-work-early/ [Accessed 18 Jun. 2018].

NOTE: 12 September 2019 
The timing of the strikes indicates that the planes were electronically hijacked.
Note 05 November 2019
The biggest clue that the attack was designed to minimize casualties is that they did not set the bombs off until after the people were evacuated from the buildings.