Freeway operations need metering, two phase signals, free entry, road pricing and density control for efficiency and safety.

Submitted by john on Mon, 16/12/2019 - 16:09

While the process described above is proposed to craft green waves on arterial roads, it has limited application for freeways. Melbourne’s freeway and tollway network carries 30% of the arterial road traffic although comprising only 7% of the arterial road network length. Progress is being made on managing freeway operation to achieve optimum speed, without congestion, and to establish reliable trip times.

Ramp metering to control freeway traffic has proven successful as described in VicRoads’ Freeway Ramp Metering Handbook. The method uses occupancy as a criterion to limit inflows and avoid congestion. Managing varying entry flows across adjoining ramps can spread the micro peaks, decrease freeway turbulence, and increase freeway capacity. Congestion on freeways leads to flow breakdown, loss of throughput, consequent increased delays, and should never be permitted by the control strategy, but this is not consistently achieved.

The need to prevent crashes is even more important for all safety, delay and reliability aspects. Contemporary understanding of freeway crashes is rapidly developing, and it is considered that two classes of freeway crash are prevalent: those at medium density, at moderate speed, associated with lane changing, with serious consequences, described as “swooping”; and those at high density, at low speed, with minor consequences, and described as “rear-end”. Control systems seek to avoid occurrences of high densities. Limiting the exposure to swooping is under development, but is restricted by sub-par observation equipment. Speed control is used on freeways related to traffic incidents, including roadworks, and care is taken with the rate of change of speed and associated densities to avoid unsafe states.

Professor Boris Kerner has defined a 3-phase model of traffic flow that better explains observed traffic behaviour than the widely accepted 2-phase model. Not to be confused, the two phase model of traffic flow relates to the speed, volume, and density of traffic flow; but 2-phase operation of signals is the number of primary signal phases at an intersection. John Gaffney and Matthew Hall of VicRoads have observed freeway traffic behaving according to the 3-phase model, including nucleations and associated road crashes, derived from TIRTL measurements at 500m intervals on the Monash Freeway. Regulation of swooping needs to be invoked. Refer also to John Gaffney’s Churchill Fellowship report

Ramp metering already makes provision for high priority vehicles to jump the entry queue, but it needs to be further developed to include a priority toll, as is proposed above for arterial streets, to permit as many vehicles as possible to jump the queue, where the queue delay or queue length is significant.
Allowing ~90% traffic to use the priority lane for a small toll, reduces the ramp meter queue length by ~90%, increases the queue delay slightly for the 10% free entry queue, slightly reduces the demand, reduces the queue CO2 emissions by ~90%, and reduces the average delay by ~90%. Both the priority and queued lanes need to be metered, but the priority lane should normally flow freely. This 90% reduction in queue length is important where the ramp meter queue is 1.5km long, and jumping is important when the queue delay is 30 minutes.

Clearly, 2-phase intersections at the freeway exits, and downstream, will increase freeway capacity where the exit capacity is a constraint.

Individual vehicle behaviour cannot be observed in detail from the TIRTL measurements. Jeff Hecht writes that: automotive lidar remains in flux; long-range microwave radar works better in bad weather; ultrasound is best for parking assist; short-medium range radar is good for cross traffic, rear-end collision and blind spot detection; optical cameras have good resolution and can detect traffic signals; and lidar can directly measure distance and speed for objects up to 300m. An example from Luminar lidar is shown, but it is likely that a combination of a few technologies can best observe traffic and will come from the car industry.

VicRoads practice is to reduce speeds in 10kph increments during incidents because of road safety. The Police practice of stopping vehicles on freeways, requiring a 60kph speed change increment is therefore relatively dangerous and should be avoided.

This site does not make all necessary prescriptive conclusions on freeway operation.