LEFT LANE LAWS CUT ROAD CAPACITY

Laws Reserving the Left Lane for Passing
Can Cut Highway Capacity in Half



constriction ahead Too many legislators with 1920's brains still think that the right lane is for driving and the left lane is for passing. They selfishly want to make other drivers leave room for them to pass other cars. But they will just delay everyone with their foolishness.

Traffic flows best when all cars are going the same speed and nobody is passing. But too many drivers and politicians want the power to selfishly get ahead of other drivers. As we shall see, every car that passes another car reduces the overall capacity of the highway.



ROAD AND HIGHWAY CAPACITY

How many vehicles per hour can a traffic lane carry? The following table shows the numbers.

NORMAL SPEED CURB PARKING LANE POSITION ONE-WAY CAPACITY TWO-WAY CAPACITY NOTES AND REASONS
≤ 45 mphYesRight 1300500 Subtract 500 veh/hr if lane next to parking lane
Center 18001000 Center lane is not next to parking lane
Left 13001000 Two-way left lane is not next to parking lane
NoRight 18001000 One-way is set by safe following distance
Center 18001000 Two-way subtract 800 veh/hr for oncoming left turns
Left 18001000 Two-way left lane is lower due to waiting left turns
> 45 mphRight 1200667 High speed only 2/3 for larger following distances
Center 1200667 Center lane never reserved for passing
Left 1200667 Lane reserved for passing would have no capacity

Notes:
- 2-second minimum safe following distance is used to get the 1800 veh/hr value.
- 3-second minimum following distance is needed above 45 mph, giving 1200 veh/hr.
- Two-way traffic capacity is reduced by left turns across oncoming traffic and waiting to turn.
- Use one-way capacity if the road is divided and the median has no openings.
- On-street parking reduces the capacity of the adjacent lane by 500 veh/hr.
- Multiply lane capacity by the portion of time the traffic signal is green to get the capacity through the signal.
- If two-way double-alternate signal progression is used, divide the signal capacity by 2.
- Roundabouts have a capacity of 1800 veh/hr total of all approaches.
- All-way stops have a capacity of 1500 veh/hr total of all approaches.

When traffic exceeds the capacity of the road, the speed goes way down to maintain safe following distances.

CAPACITY WHEN NOBODY IS PASSING

How many vehicles per hour can use a one-way highway when no vehicles pass other vehicles?

SPEED1 LANE2 LANE3 LANE
≤ 45 mph180036005400
> 45 mph120024003600

Each lane can carry 1800 veh/hr at speeds not above 45 mph.

Each lane can carry 1200 veh/hr at speeds above 45 mph.

CAPACITY REDUCTION CAUSED BY ONE PASS

How many vehicles per hour can use a one-way highway when one vehicle passes one other vehicle in that hour?

How much space is needed to let one extra car enter the stream to pass?
At least enough space for one car and its following distance is needed.

How much space is needed to let one car re-enter the original stream?
At least enough space for one car and its following distance is needed, since the original space vacated by the passing car is behind it.

Room is needed for one extra car in each lane involved to allow one car to pass.

SPEED2 LANES3 LANES
≤ 45 mph35995399
> 45 mph23993599

Each car that changes lanes needs an empty space for one car to fit into in the lane it moves into.

In heavy traffic, the traffic in the other lane must make room for the passing car to enter the lane. This slows all of the traffic down.

CAPACITY WHEN ALL DRIVERS CONSTANTLY CHANGE LANES

How many vehicles per hour can use a one-way highway when all vehicles are constantly changing lanes?

How much space is needed to each car change lanes constantly?
At least enough space for one car and its following distance is needed for each car.
This assumes that all of the cars change lanes at random.
For all of the cars in the right lane to move to the next lane, either both lanes have only 900 cars each, or the next lane has enough space to take all of the cars in the right lane. If the right lane has 1800 cars, the next lane must be empty.

Room is needed for one extra car in each lane for each car in the lane.

SPEED2 LANES3 LANES
≤ 45 mph18002700
> 45 mph12001800

There must be an additional empty space for each car on the road.

Whenever you pass a car in heavy traffic, it delays all of the cars behind you.

Whenever someone in front of you passes in heavy traffic, it delays you.

Whenever someone in front of you passes a vehicle going below traffic speed, nobody is delayed because a gap opens ahead of that vehicle.

In heavy traffic, everyone would get there faster if nobody on the road passed anyone else (other than vehicles going below traffic speed.



When the highway is near its capacity, every lane change or pass slows down the traffic behind it to make room for it to happen. This makes it take longer for everyone to get where they are going.





In heavy traffic, each driver passing a car up ahead of you slows you more than you gain by passing a car.



CAPACITIES WITH AND WITHOUT THE LEFT-LANE LAW

SPEED LEFT LANE
LAW
2 LANES3 LANES
LEFTRIGHTTOTAL LEFTCENTERRIGHTTOTAL
≤ 45 mphNO 180018003600 1800180018005400
YES 018001800 0180018003600
> 45 mphNO 120012002400 1200120012003600
YES 012001200 0120012002400

Notice that the reduction of capacity caused by the left-lane law is equal to the capacity of the left lane.

This is because, when nobody is passing, all of the traffic must be in the other available lanes. This means that ALL of the traffic must fit into the other lanes. So the left lane is effectively removed from the capacity of the highway.



Reserving a lane for passing effectively removes that lane from the traffic capacity of the road.



When nobody is passing, all of the traffic must fit in the other available lanes. This law requires vehicles to stay out of the left lane unless they are passing or turning left.

Question: What happens when one car passing another comes upon a car that is turning left (or taking a left exit)? It definitely spoils the pass.



Those who want a lane reserved for passing are selfish enough to want the vehicles in the left lane (usually 1800 vehicles per hour) removed from the road so there is enough room to pass other cars.



Where are those 1800 cars supposed to go?

EFFECT OF THE LEFT-LANE LAW ON A MERGE

After a heavy merge, there may not be enough room for all of the vehicles on the highway to obey the left-lane law. Traffic on the road may exceed the calculated capacity above.

Those cars will fit on the road, but they are breaking the left-lane law to do it.



Those who want the left-lane law say it increases road capacity. This is false. It actually reduces the capacity of each multilane highway by the capacity of the left lane (usually 1800 vehicles per hour) in each direction.



It's time to get rid of the idea that passing other vehicles is important. Every time someone else passes in heavy traffic, it slows you down.