Automated driving has the potential to significantly increase highway capacity by allowing smaller gaps between vehicles and more narrow lanes. Current highway capacity is limited by human driver performance, but automation can remove the human from the control loop. Partial automation with cooperative adaptive cruise control (CACC) could improve vehicle following dynamics and reduce gaps. Full automation without any human involvement could enable even smaller gaps and lane widths, allowing highway throughput to reach 2,200 vehicles per hour per lane and occupancy rates of only 5% of the road surface.

1. 1
Highway Capacity Increases
From Automated Driving
Steven E. Shladover
California PATH Program
July 25, 2012

2. 2
Capacity increase opportunity
• Today’s highway capacity governed by
human driver performance limitations
• Replacing driver with automation can
increase capacity by:
– Shorter vehicle-following gaps
– Enhanced vehicle following stability IFF
automation is cooperative (eliminating
shock waves)
– Narrower lanes for light duty vehicles
based on more accurate steering

3. 3
The Highway Capacity Challenge
• At maximum throughput (2200 veh/hr/lane),
vehicles occupy only 5% of road surface
 Half the lane width for a
full-size car or SUV
 Average longitudinal
gap = 9 car lengths
 2200 veh/hr = 1.64 s hwy
 At 60 mph (~100 km/h),
this is 144 ft per vehicle
(44 m or ~10 vehicle
lengths)

4. 4
Capacity Increase Opportunities
• Partial automation – cooperative ACC to
improve vehicle following dynamics and
reduce gaps
• Full automation – driver removed from
control loop, enabling operations outside
driver response capabilities

5. 5
Mean CACC/ACC Time-Gaps Selected
in Vehicle Following (Test Results)

6. 6
Simulated Lane Capacity vs. CACC Market Pen.
With addition of Vehicle Awareness Devices

7. 7
NAHSC “Pipeline” Capacity Estimates
– Individual Automated Vehicles
Autonomous Highly Cooperative
Achievable capacity will be ~25% less to allow for merging and
lane changing

8. 8
NAHSC “Pipeline” Capacity Estimates
– Platooned Light-Duty Vehicles
vs. Platoon Length vs. Intra-Platoon Gap
Achievable capacity will be ~25% less to allow for merging and
lane changing

9. 9
Automated Truck Platoon Capacity
• NAHSC studies (1997)

10. 10
Traffic Flow Stability
• Drivers’ car-following response lags cause
shock wave instabilities (“stop and go”)
• Commercially available autonomous ACC
systems can have comparable or worse lags
• Cooperative ACC dramatically reduces lags
and improves stability
• Cooperative fully automated systems can
also eliminate flow instabilities