During weekdays the population of Manhattan grows to 3,940,000 people[1]. This population consists of 1,460,000 local residents, 1,610,000 commuting workers, 374,000 local day-trip visitors, 70,000 commuting students, 404,000 out of town visitors, and 17,000 hospital patients[1]. If we add the workers and students with the local day-trip visitors we find that 2,060,000 people need to enter and exit Manhattan around the same times each day. Aerial Manhattan Currently the majority of these trips are provided by transit, with only 16% of people commuting by personal vehicle[2]. So how would things be different if Manhattan was not served by transit and everybody drove instead? BRIDGES AND TUNNELS The first thing to consider is the road space available for vehicles entering and existing Manhattan. The existing automobile lanes provided by the 20 bridges and tunnels connecting Manhattan are counted in Table 1.

Table 1. Automobile lanes on Manhattan crossings
INBOUND OUTBOUND REVERSIBLE
1. Brooklyn Battery Tunnel[3] 2 1 1
2. Brooklyn Bridge[4] 3 3
3. Manhattan Bridge[5] 2 2 3
4. Williamsburg Bridge[3] 2 2 4
5. Queens Midtown Tunnel[3] 2 1 1
6. Queensboro Bridge[3] 4 3 2
7. RFK Bridge[6] 3 3
8. Willis Av Bridge[7] 0 4
9. 3rd Av Bridge[8] 5 0
10. Madison Av Bridge[9] 2 2
11. 145th St Bridge[10] 4 4
12. Macombs Dam Bridge[10] 2 2
13. Alexander Hamilton Bridge[11] 4 4
14. Washington Bridge[12] 3 3
15. University Heights Bridge[10] 2 2
16. Broadway Bridge[10] 2 2
17. Henry Hudson Bridge[13] 4 3
18. George Washington Bridge[14] 7 7
19. Lincoln Tunnel[15] 2 2 2
20. Holland Tunnel[16] 2 2
TOTAL 70[17] 65

A freeway lane operating under ideal conditions can transport 2,000 vehicles per hour[18]. From this we can determine how many vehicles per hour could enter and exit Manhattan under ideal conditions, and the amount of time for 2,060,000 vehicles to do that. The results are presented in Table 2 below.

Table 2. Flow rate and travel time for potential vehicles on existing Manhattan crossings (assuming ideal traffic flow)
INBOUND OUTBOUND
Hourly Vehicle Capacity 140,000 130,000
Total Time For All Vehicles To Enter/Exit 15 hours 16 hours

This situation would be problematic because it leaves less than no time for other activities. In order to make this scenario function we will assume enough capacity is needed for all vehicles to enter Manhattan within a 4 hour AM peak period and exit within a 4 hour PM peak period. This would require 380 additional traffic lanes, a 280% increase, which could be provided by 48 new 8-lane crossings.[19]

Manhattan Crossings

48 arbitrarily located lines representing additional 8-lane crossings

PARKING The other thing to consider for this auto-oriented Manhattan is the additional parking required. The 2,060,000 people driving into Manhattan need at minimum one parking space each. With 30m2 [20] required for an off-street parking space this adds up to a total of 62km2 of parking. Manhattan is 60km2 [21] so this is equivalent to a layer of underground parking under the entire island. If we assume the local residents drive to get around and own vehicles at the US average of 82 vehicles per 100 persons[22] that will result in 1,200,000 more vehicles in Manhattan. This means 2,270,000 parking spaces if each vehicle has a space available at home and one available at work (130,000 Manhattan residents work elsewhere [1]). In total a very conservative estimate for the amount of parking required is 2 layers underneath all of Manhattan.

NOTES AND REFERENCES 1. Moss, M., & Quing, C. (2012). The Dynamic Population of Manhattan. p. 1. 2. Moss, M., & Quing, C. (2012). The Dynamic Population of Manhattan. p. 13. 3. Greenberger, D. https://groups.google.com/forum/#!msg/misc.transport.road/5a0SbUiJeyg/bIAzwePLcgIJ 4. http://en.wikipedia.org/wiki/Brooklyn_Bridge 5. http://en.wikipedia.org/wiki/Manhattan_Bridge 6. http://www.nycroads.com/crossings/triborough/ 7. http://www.nycroads.com/crossings/willis-avenue/ 8. http://en.wikipedia.org/wiki/Third_Avenue_Bridge_(New_York_City) 9. http://www.nycroads.com/crossings/madison-avenue/ 10. http://www.nyc.gov/html/dot/html/infrastructure/bridges-harlem.shtml 11. http://www.nycroads.com/crossings/alexander-hamilton/ 12. http://www.nycroads.com/crossings/washington-heights/ 13. http://www.nycroads.com/crossings/henry-hudson/ 14. http://en.wikipedia.org/wiki/George_Washington_Bridge 15. http://en.wikipedia.org/wiki/Lincoln_Tunnel 16. http://en.wikipedia.org/wiki/Holland_Tunnel 17. TOTAL = INBOUND + REVERSIBLE 18. Indevelopment (2004).Road Capacities. p. 9. 19. REQUIRED LANES = TOTAL TRAFFIC / LANE TRAFFIC FLOW / NUMBER OF HOURS        REQUIRED LANES (INBOUND) = 2,060,000 / 2,000 / 4 = 258        TOTAL REQUIRED LANES = 258 x 2 = 516        TOTAL ADDITIONAL LANES = 516 – (65 + 70) = 381        381 / 8 = ~48 20. Shoup, D. (2005). The High Cost of Free Parking (p. 221). 21. http://en.wikipedia.org/wiki/Manhattan 22. U.S. Census Bureau (2010). State and Metropolitan Area Data Book: 2010. p. 79.