Headway

The minimum headway is the shortest such distance or time achievable by a system without a reduction in the speed of vehicles.

On the other end of the scale, a system with short headways, like cars on a freeway, can offer relatively large capacities even though the vehicles carry few passengers.

In principle, automated personal rapid transit systems and automobile platoons could reduce headways to as little as fractions of a second.

Where vehicle size varies and may be longer than their stopping distances or spacing, as with freight trains and highway applications, tip-to-tail measurements are more common.

The most common terminology is to use the time of passing from one vehicle to the next, which closely mirrors the way the headways were measured in the past.

Trains can only accelerate and decelerate relatively slowly, so stopping from anything but low speeds requires several hundred metres or even more.

Signalling systems serve to provide drivers with information on the state of the track ahead, so that a collision may be avoided.

An ABS system divides the track into block sections, into which only one train can enter at a time.

This is why train headways are normally measured as tip-to-tip times, because the clock was reset as the engine passed the workman.

This was an important consideration for the Advanced Passenger Train in the United Kingdom, where the lengths of block sections limited speeds and demanded a new braking system be developed.

[5] In the case of automobile traffic, the key consideration in braking performance is the user's reaction time.

That means that the driver will be matching their speed to the vehicle in front before they reach it, eliminating the "brick-wall" effect.

Widely used numbers are that a car traveling at 60 mph will require about 225 feet to stop, a distance it will cover just under 6 seconds.

Nevertheless, highway travel often occurs with considerable safety with tip-to-tail headways on the order of 2 seconds.

Whether traditional headway regulations should apply to PRT and car train technology is debatable.

Even when the locomotive applies emergency braking, the cars following do not suffer any damage because they quickly close the gap in the couplings before the speed difference can build up.

There have been many experiments with automated driving systems that follow this logic and greatly decrease headways to tenths or hundredths of a second in order to improve safety.

Today, modern CBTC railway signalling systems are able to significantly reduce headway between trains in the operation.

Using automated "car follower" cruise control systems, vehicles can be formed into platoons (or flocks) that approximate the capacity of conventional trains.

[11] The headway, in turn, is defined by the braking performance, or some external factor based on it, like block sizes.

Following the methods in Anderson:[12] The minimum safe headway measured tip-to-tail is defined by the braking performance:

where: The vehicular capacity of a single lane of vehicles is simply the inverse of the tip-to-tip headway.

For comparison, the Marin County, California (near San Francisco) states that peak flow on the three-lane Highway 101 is about 7,200 vehicles per hour.

Notwithstanding these formulas it is widely known that reducing headway increases risk of collision in standard private automobile settings and is often referred to as tailgating.

2) metro system, per line: 40 km/h (~11 m/s) speeds, 1000 passengers, 100 meter vehicle length, 0.5 m/s^2 braking, 2 second reaction time, brick-wall stop,

of 1.5; Note that most signalling systems used on metros place an artificial limit on headway that is not dependent on braking performance.

For this reason, the London Underground system has spent a considerable amount of money on upgrading the SSR Network,[14] Jubilee and Central lines with new CBTC signalling to reduce the headway from about 3 minutes to 1, while preparing for the 2012 Olympics.

However, these systems are often constrained by brick-wall considerations for legal reasons, which limits their performance to a car-like 2 seconds.

In this case: Headways have an enormous impact on ridership levels above a certain critical waiting time.

Following Boyle, the effect of changes in headway are directly proportional to changes in ridership by a simple conversion factor of 1.5.