Congestion? What?s Congestion?

Today’s article is the fourth in this blog’s five-part series this week on transport and our streets. It’s a contribution from our Queen Edith’s city councillor Tim Moore, who has consistently been involved in transport issues since he was elected in 2014. While I don’t necessarily agree with Tim on every point, I’m impressed that one of our councillors is thinking things through so thoroughly, and I’m delighted to give his ideas wider coverage. – Chris

An overview of Greater Cambridge transport infrastructure issues

By Dr Tim Moore BA Mod (Hons) MB BCh BAO DCH MSc MA FFPHM
City Councillor, Queen Edith’s Ward



  • Queues of cars kilometres from the Cambridge on radial routes into the City, and on most roads through roads within the City during peak travel time
  • Long and lengthening journey times, for longer periods each day
  • Air quality deteriorating within the City from queueing vehicles
  • Bus services in the City and Greater Cambridge are particularly slow in peak times, with inadequate route coverage, costly and with few passengers much of the day


  • Capacity of road network is inadequate for peak demand times (journeys per hour)
  • Increasing queue lengths eventually impede traffic throughput, and increase journey times. When more vehicles arrive at a bottleneck each minute than can get through, the traffic queue backs up to interfere with surrounding junctions and bottlenecks. Once this threshold is reached the capacity of the network to carry vehicles drops fast leading to gridlock.
  • Road network is rationed passively by allowing queues to form ? a grossly inefficient means of rationing a scarce and valuable resource
  • Causes high cost to the City for business and social functioning, and is unsustainable
  • City Deal solutions proposed will worsen the situation
    – Longer queues outside City centre, so more air pollution and slower travel
    – Unnecessary environmental damage: bus only roads on green belt, trees sacrificed for bus lanes, total bus journey times still poor except in City centre
    – Reduces the network carrying capacity by pruning and limiting routes


  • A paradigm shift from private to public transport (and other modes of travel)
  • Active management of road network to optimise journey times and maximise throughput for road users and buses through charging
  • Rationing road use through charging vehicles to use each bottleneck or sit in a queue, with an increasing tariff per minute as journey times increase
  • Charges managed to ensure the road network works optimally, the charges being used to subsidise public transport and park & ride locations
  • Self adjusting variable charges linked to using each bottleneck in real time
  • A non-profit organisation managing an integrated transport (bus) network. Road charging used to keep bus fares lower (during peak times at least), buses fuller, with integrated ticketing. Cost neutral solution for all organisations involved.


Congestion is an inevitable consequence of any bottleneck in a network: in a road network this includes junctions, lights, road works, parked vehicles etc. It is only important because:

  • Air pollution is made worse by queuing vehicles: important because this has health consequences, for example particulates from diesel vehicles are a serious cause of disease and shortened lives. Children and elderly are particularly vulnerable.
  • Time is wasted in queues and time is money: the cost to business in the City is very considerable, for example tradesmen stop earning, and carers can?t help elderly out of bed, chief executives and professionals are not productive, nobody knows how long it will take to reach their destination.

Congestion is a byproduct of increasing traffic as it approaches the road network?s maximum carrying capacity measured in journeys per hour, network throughput.

Network congestion is?the result of rising journey times as more vehicles enter the road network. The relationship between the number of additional vehicles and the increase in travel time is complex and network specific. We know that when there are few vehicles using the network, a few more make almost no difference to travel time. The nearer the network is to its maximum carrying capacity (A), the greater the increase in journey time for each additional car. A critical point is reached (B) after which gridlock occurs rapidly.

Network throughput (journeys per hour) is the critical factor that needs to be maximised. The aim is the largest number of journeys possible per hour while keeping travel time and congestion reasonable.

The network capacity is a limited and valuable commodity. How can we optimise its use?

  • Ration by queueing: the current situation is the most costly solution of all as, beyond point (B), journey time increases rapidly and vehicle throughput falls as the network fails. Pollution increases and valuable time is lost. How much is this costing the City?s health and economy?
  • Active network management, managing the number of vehicles using the road network at any time is the optimal solution. This means limiting the number of vehicles using the road network to keep travel time fast and queues to a minimum.


Variable road use charging depending on current/predicted congestion levels, charges rising rapidly to deter all but those needing to pay a high deterrent price, whilst allowing individuals freedom to choose to pay, provides the optimal solution.

  • To charge road users, as or before travel time starts rising (A), an increasing road charge to (B) and punitive beyond (B) to deter new entrants with very high charges (see graphs above).
  • A fixed charge could be used at other specified times to ensure a bus subsidy is present during these times.
  • Fixed London type congestion charges raise funds but don?t prevent or manage network congestion.
  • It is not possible to fine tune a generic charge depending on which bottlenecks are used (scarce resource) and the effects this is having on travel times, and the traffic at that time.

Will other forms of ?congestion charge? work? Only if they ensure that:

  • The entire local road network throughput is maximised (and so gridlock/ slow-down is prevented) ? a self adjusting mechanism that reacts to early signs of network overload (congestion building up)
  • Funds (from charges) can guarantee sufficient parking places and bus services to maintain the switch from cars during peak road network demand, and deal with increasing pressure on this network as population and jobs increase. Light rail, tunnels and other new public transport services may be needed in the longer run as well as road improvements around the City.
  • The net cost of building, maintaining and managing the active network management system is cost neutral to the public purse (or is balanced with subsidies from private or public sources)

Fixed charges alone cannot do this ? the charges need to become so great (when required in real time) that a sufficient proportion of potential road users opt for alternatives once close to the City. Buses must be available, reasonably priced, with integrated public transport network when needed ? to be a reliable alternative. That way the buses can be guaranteed fast reliable journeys.

The alternative is to provide a costly parallel network (unavailable to private vehicles) of bus only lanes and roads, leaving the public roads gridlocked for periods in the day. This is wasteful of public funds, environmentally destructive, and highly costly to the business of the City.

Any action that reduces the network capacity will reduce network throughput and so is counterproductive: this includes most bus lanes, closed roads (deemed ?rat runs?), segmenting the City, and penalty charge zones (excluding most vehicles) for fixed periods of the day.

Practical consequences

The City is entering a paradigm shift away from free use for all vehicles at peak times: practical alternatives are required, and a fair vehicle charging structure that relates to use of scarce network resource (network bottlenecks in particular). For this to work a functional (much improved) public transport network needs to be in operation as road charging is implemented. The aim is that all travellers can get to their destination within a reasonable and repeatable time, when they need to, using the best mix of transport modes and at a reasonable cost. Those who choose can pay a larger price for the use of limited road capacity may do, but the charge must be sufficient to deter as travel time starts to rise (and throughput falls).

An increasing number of travellers to Cambridge are coming from surrounding areas and further afield. Many have no real alternative but to use their car, at least to reach Park and Ride outside the City (see Zone A below). Within the City it is not currently possible to get to a variety of locations with public transport.

Alternatives to the car should be encouraged and provided:

  • Safe walking and cycling: good cycle-free pavements especially important for the elderly, disabled and young, and cycle paths fit for both slower and speedy users and safe for school goers.
  • Integrated public transport network: a mix of buses, trains, and other possibilities such as light trains.
    – Sufficient public parking well outside the City (Zone A), Park and Ride (P&R) sites are ideal if sufficient capacity is available,
    – P&R acting as public transport hubs, fed by longer distance buses, +/- light rail, allowing a frequent and speedy onward journey on non-polluting public transport or on cycle paths or on foot
    – with integrated parking and public transport ticketing, and within-City hub(s) to reach all parts of the City


Vehicle charging

Users need to know:

  • Likely travel time and charges for getting to their destination by car. An app, given their destination, could estimate charges from live data and recommend the least congested (cheapest) route. A live map could also provide estimated costs to a fixed set of destinations from each Park and Ride. Public car parks in the City should be included, with their charges listed;
  • Likely travel time on public transport, and which to use. An app could provide live routing (with bus interchanges) and time information;
  • City residents would face the same charges as those entering the City once they left their drives. It is a fair approach.

Financing the changes

An economic perspective might be to invest a sum such that the cost and benefit are approximately equal. The starting point is an economic evaluation of the current and projected cost to business and health. Where is this?

The City is wealthy and growing and offers viable business opportunities for investment in its public transport, alongside public funding. Where is the overall financial plan, including private and institutional investment?

There is a partial plan driven by City Deal monies, and perhaps two private bus companies providing limited services to some parts of the City. Disjointed incremental planning in silos is a costly and a risky means of delivering the required paradigm shift. Penalising road users when no alternative is available is foolhardy and economically damaging.

Managing the public transport network

An integrated public transport network requires an integrator, a non-profit organisation to which the key elements of the public transport network including the City and County Councils, Universities, existing bus companies and other (potential) private investors belong. Given current austerity plans, it is required that public assets are run in a cost neutral manner (Park and Ride hubs, capital and running costs). Integrated ticketing across the various service providers is highly desirable. The integrator organisation needs to plan and manage the expansion of the public transport network and share some financial risks. Where is this organisation which should be in planning mode now?

As economies of scale are achieved the costs of running fuller buses are lower and so the cost to each user should fall. Bus fares should therefore become cheaper in the long run.

For many routes smaller buses running predominantly at peak times are needed (for example school and employee pick up in villages without a bus service, with delivery on a radial route into the City). Within City frequent buses from the bus interchanges (including Park and Ride) and Railway stations will be required. Smaller less polluting buses may prove viable. High capacity buses are best suited to longer distance radial routes.

Traffic sensors to enable charging

Public funding of sensors is a first step, ideal for City Deal funds. Number plate recognition (NPR) works with a London style congestion charge perimeter and where the number plate is visible (e.g. car entering a bus lane). It may be less useful for road charging unless each bottleneck has NPRs. Roadside sensors and responsive vehicle tags may be less expensive to install. A live app reporting back to the charging mechanism in real time may also be possible. Sensors are required at bottlenecks (points of congestion) on the roads leading into Cambridge, both outside and inside the City perimeter (including the City centre) and perhaps at other points in the Greater Cambridge area where travel time rises during peak times.

In Summary ? The Pickle

So many of the required elements for an optimal solution are missing, some planned changes are simplistic destructive and retrogressive. There is no master plan, poor vision, and a destructive set of City Deal projects. Where is the change of paradigm plan? We don?t even know the business and health costs to the greater Cambridge of current and planned road rationing models. In a City with such skills, abilities and wealth at hand, a practical futuristic vision should be deliverable. Where is the vision in the current plans, the joint planning, investment, and risk sharing? What is the organisation charged with ensuring the integrated public transport system operates within the funding available?

Thoughts on a Charging Algorithm

Purpose: to maximise the economic benefit of the whole road network (around Cambridge) by

  • Maintaining the highest throughput of vehicles in real time

    – Minimise queuing (congestion), maximise the speed of travel through the network. Many more vehicles able to benefit from the road network;
    – Prevent network degradation (observation, planning ahead and learning function);
    – Satisfying the largest number of road users;
    – Using real time location specific user charging to ration use of bottlenecks;
    – Deterring vehicle drivers by showing predicted road charge for journey and cost for alternative mixes of buses, train, Park and Ride, walk and cycle.

  • Maintaining the highest throughput of people in real time by providing an integrated bus network, fuller buses
  • Providing a routing app for individuals to use that distributes traffic to avoid slower bottlenecks in real time (evens the network load) when given a destination
  • Manage each limiting segment of the road network by charge to use, cost rising rapidly to ensure demand does not exceed capacity.

Tim Moore can be contacted by email at

5 Replies to “Congestion? What?s Congestion?”

  1. Really excellent article, Tim (and you know I don’t give praise too readily). If only those making the decisions that matter to this city had applied even half of the analytical power captured in this one article before shaping their proposals, they wouldn’t be in the absolute mess we’re now witnessing.

  2. Took me an hour to get from train station to Queen Edith’s Way this evening! No buses for over 30 mins (no. 1 & 3). Then 2 no. 1s & no. 3 arrive…..Something has to change. I suggest that everything alternate bus doesn’t go to train station but bypasses Station Road as the traffic to the station significantly slows down journey times. This is done on several routes in London and works well.
    Won’t solve everything but need to start somewhere!,

  3. Hi All seems like mafia have taken control of our councils ,deal has already been done ,you can shout till the cows come home it will go ahead whatever consultations are just a cover to save face showing ,we followed procedure .
    I was in Manchester last week got on a free bus went anywhere around centre funny how some towns can finance we a wealthy city can’t ,or is the money being used for other purposes ,maybe we should have an enquiry .?
    Solution get rid of councilors and planners they are weak and gutless don’t have Cambridge in their hearts .
    The city has been ruined to resolve the issue without disrupting business is too late little thought goes a long way ,solution have a one way system around town this would create space offering two lanes in one direction ,this I believe is the only answer instead of squeezing bus lanes ,cycle routes ,ambulances and fire engines down one lane .

  4. Took Me 45 Min last night to get to Catholic church from Blinco Grove ,my main concern is scenario if someone has a heart attack or any emergency in their vehicle ,access to the vehicle would be impossible ,also emergency services how will they get through ,there is not enough vehicular space now to get through ,this is total madness instead of easier travel it has now become complicated SOMETHING MUST BE DONE ? can I suggest one way traffic

  5. This is an excellent addition to the discussion. Variable pricing on motorways, with exceptions for High Occupancy Vehicles, has long been in use in California and without the benefit of apps and mobile phones which can add to its utility.

    Driving out of Cambridge at 7.30am last Wednesday on Hills Road, the traffic queued back to the Gogs. The weather was dry and it was daylight, so no additional reasons other than traffic level, to cause the delay. Clearly the strategy of “I’ll head in earlier to miss the traffic” is no longer an option showing that demand is reaching capacity.

    Perhaps what is needed is a consultation period where the best, most practical proposals are sought and discussed and then the 5(?) best, including perhaps the ‘do nothing option’, are put to a vote with a transferable voting system. Then hopefully we would reach a consensus and avoid a continual ‘not this’ situation.

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