He Credited My Traffic Flow Optimization to His Own Infrastructure Strategy — Then the Funding Audit Required My TPP Accreditation
The desire line map was printed in A3 format.
It was lying flat across the wide drafting table in the Municipal Transport Directorate office.
The map showed the origin-destination trip patterns for the eastern corridor.
The solid blue arrows represented the existing transit travel volumes between the residential zones and the city center.
The dashed red lines represented the projected rail ridership from the rail extension proposal.
Amara had drawn circles in red ink around three major development zones on the eastern boundary.
She had written: “Local Plan conflict — competing retail sites” in her small, compact handwriting between the arrows.
Junior modeller Kofi was sitting at the secondary monitor, sorting through the trip generation matrix files.
He was twenty-six.
He had spent three weeks distributing the travel survey datasets under her direction.
He had calibrated the gravity model parameters and aligned the zone classification variables.
He was precise.
“Kofi,” Amara said.
Kofi turned from the screen.
She pointed to the red circles on the map.
“The mode choice model was using a five-year-old baseline dataset,” she said.
“The Local Plan committed those eastern development zones in 2022.”
“The existing business case doesn’t account for them.”
“The competing retail zones will capture approximately twelve thousand trips from the rail transit corridor.”
“The trips will go to highway vehicle travel, not the rail metro extension.”
“The authority’s projection was fifty-three thousand six hundred daily boardings.”
“Our model forecasts forty-one thousand two hundred.”
Kofi looked at the red circles.
“That reduces the ridership projection by twenty-three percent,” he said.
“Yes,” Amara said.
“And the twenty-three percent reduction changes the cost-benefit ratio.”
“At forty-one thousand two hundred, the project falls below the DfT threshold for unconditional funding approval.”
“The business case cannot support a direct Treasury grant.”
“It will require a value-for-money commitment review and a conditional funding path.”
“The model must follow the WebTAG guidelines to be compliant.”
She opened the model repository directory on the server.
She logged the final trip distribution run logs.
The file header carried her CTPP registration signature code: CTPP-TD-AS-4416.
She rolled the A3 desire line map.
She placed it inside the black plastic project tube.
She set the tube on the shelf behind her desk.
It would stay there as the reference record for the Eastern Metro demand forecast — the physical proof of the 23% overstatement.
—
She read the Treasury Green Book submission confirmation four weeks later, in her office.
The document was titled: “Gould Demand Forecast — rail demand re-modelling conducted under Chief Transport Planner J. Gould’s direction.”
She scrolled to the third page.
“Transport modelling support: Dr. Amara Sesay, CTPP-TD-AS-4416.”
She looked at the black project tube on the shelf.
The desire line map was rolled inside.
The red circles.
The “Local Plan conflict” text.
She opened the SATURN model database.
She checked the run directory.
Ridership forecast: 41,200.
Original projection: 53,600.
She closed the database.
She opened the next model brief on her screen.
—
Three weeks before the submission was filed, Chief Transport Planner Dr. James Gould had come to her office.
He was fifty-seven.
He managed the directorate’s infrastructure appraisal programme and held the signatory authority for all DfT technical submissions.
He had taken the desire line map from her desk.
He had unrolled it.
He looked at the blue arrows.
He looked at the red circles.
He looked at the “Local Plan conflict” text.
She said: “The four-stage WebTAG model is the necessary baseline.”
“The fifty-three thousand six hundred projection is biased because the gravity model assumed the old land-use baseline.”
“The DfT will reject the business case if they perform an independent mode choice audit.”
“The Green Book submission must specify the Local Plan adjustments to justify the funding path.”
“The model is certified under my CTPP registration.”
He held the map.
He said: “This is the honest forecast the Green Book appraisal needs.”
She said: “The submission carries my CTPP registration, CTPP-TD-AS-4416.”
“The registration should be cited in the Competent Person declaration for the transport module.”
He said: “Good work, Amara.”
He set the map back on her desk.
He left.
She looked at the map.
She noted: “the Green Book appraisal needs.”
The modelling had made it visible.
Her modelling.
She opened the CIHT registration portal.
She verified her registration details.
CTPP-TD-AS-4416.
Chartered Transport Planning Professional.
Amara Sesay.
She closed the browser.
She went back to the SATURN calibration files.
The Eastern Metro Extension was a proposed seven-mile rail link designed to connect the eastern suburbs to the metropolitan financial center.
The project had been budgeted at two point one billion pounds.
The initial business case had been compiled by the external consultants in London.
For three months, the municipal planning board had been using the consultants’ ridership model to justify the funding request.
The board had celebrated when the initial report projected fifty-three thousand six hundred daily boardings.
It was a high, economically viable figure that would guarantee the Treasury grant.
It looked like a certain approval.
Amara had looked at the Local Plan land-use revisions on the third day of the modeling audit.
She had noted that three major retail parks had been permitted in the eastern zone, within two miles of the proposed stations.
The local council had approved the retail permits in 2022.
But the travel consequences of those retail zones had been omitted from the consultants’ gravity model.
She had spent three days explaining to the planning director why the fifty-three thousand six hundred projection was biased.
“If we ignore the retail zones, we are assuming all residential trips will travel to the city center,” she had said.
“The retail parks will capture those trips locally.”
“The rail ridership is overstated.”
Kofi had begun the distribution matrix calibration the next morning.
It was tedious work.
They had to rebuild the trip generation tables — constructing two hundred and forty travel zones, calculating the frequency penalty for the competing highway routes, and estimating the travel time coefficients for each income group.
Every travel zone centroid had to be calibrated against observed smart card tap logs.
Kofi had verified the matrix balancing twice.
Amara had checked the trip length distribution against the national travel survey parameters.
The SATURN model had run the multi-modal assignment for eight hours.
The output was the desire line map that she now rolled into the project tube.
The Annual Congress of the Chartered Institution of Highways and Transportation was held at the Queen Elizabeth II Conference Centre in London.
Four days.
Three thousand transport planners, highway engineers, and local authority directors.
Dr. James Gould presented in the morning session of the second day.
His presentation was titled: “Calibrating Urban Rail Demand: The Eastern Metro Business Case.”
His slide 14 was her desire line map.
The blue arrows.
The red circles.
The “Local Plan conflict” annotation.
He had replaced her hand-drawn circles with a graphic designer’s clean amber borders, but the origin-destination data was unchanged.
He said: “Our WebTAG-compliant transport model identified a twenty-three percent ridership overstatement in the initial projection.”
He said: “This methodology produced a revised forecast of forty-one thousand two hundred daily boardings.”
He said: “This adjustment ensures funding integrity while aligning the project with competing corridor developments.”
He said “our WebTAG-compliant transport model.”
He did not name the four-stage demand model structure.
He did not name the CTPP Chartered Transport Planning Professional registration.
He did not name CTPP-TD-AS-4416.
He did not name Dr. Amara Sesay.
In the third row, Arthur Vance — the authority’s Director of Infrastructure Planning — watched the presentation.
He had been the one who had submitted the final Green Book business case to the DfT.
He had circulated the internal promotional booklet: “Gould’s Rail Demand Forecast.”
He had not looked at the signature page of the modeling appendix in the submission files.
He had looked at the Chief Transport Planner’s signature on the main transmittal sheet.
He watched the slide and thought: Gould has secured the appraisal credibility.
—
The NAO contact email arrived on a Thursday morning at 09:14.
The sender was Dr. Heather Lam, National Audit Office Senior Auditor.
Subject: “NAO Value-for-Money Audit — Eastern Metro Extension — Demand Model Verification.”
Amara opened the email.
“Dr. Sesay — I am writing on behalf of the National Audit Office regarding the value-for-money investigation of the Eastern Metro Extension business case. The NAO audit has identified discrepancies between the submitted rail ridership forecast and the adopted Local Plan land-use data. Under Department for Transport Green Book guidelines, all complex transport models used in funding submissions exceeding fifty million pounds must be certified by a Chartered Transport Planning Professional (CTPP). The submission identifies the demand model as ‘Gould’s Demand Forecast.’ The modeling metadata contains the CTPP registration number CTPP-TD-AS-4416. We require: one, confirmation that CTPP-TD-AS-4416 is your CTPP registration; two, the raw SATURN calibration logs and trip distribution files; three, your attendance at the audit review hearing in London. Please respond within 48 hours.”
She read “CTPP registration number CTPP-TD-AS-4416.”
She read “Green Book guidelines.”
She read “Chartered Transport Planning Professional certification.”
She looked at the black project tube on the shelf.
The desire line map was rolled inside.
The red circles.
The “Local Plan conflict” text.
She took the tube from the shelf.
She unrolled the map on her desk.
She looked at the red text.
She rolled the map.
She placed it back in the tube.
She did not call Gould.
She opened a reply to Dr. Lam.
She confirmed that CTPP-TD-AS-4416 was her registration.
She confirmed her availability for the NAO audit hearing.
She wrote: “I am compiling the complete model documentation, the SATURN calibration scripts, and the trip distribution matrices for the audit file.”
She sent the email.
She began preparing the evidence package.
The trip distribution matrices — four separate travel mode matrices.
The gravity model calibration scripts — three hundred rows of SATURN code.
The mode choice validation logs — showing the journey time and cost coefficient distributions.
Her CTPP registration certificate — showing CTPP-TD-AS-4416, valid, with the CIHT stamp.
The compilation took four hours.
She sent the complete documentation package to Dr. Lam.
She returned to her next project — a light rail feasibility study for the western docks, a different corridor, a different zone structure.
—
Gould received the formal NAO audit notification in his office that afternoon.
The email was marked high priority by the planning administration.
He read the heading: “NAO Investigation — Value-for-Money Audit — Technical Model Review.”
He read the request for the certified model files.
He called Arthur Vance.
Arthur was direct.
“The NAO guidelines are clear. A rail demand model of this scale must be certified by a Chartered Transport Planning Professional. Your highway engineering designation covers highway capacity, not multi-modal demand modeling. You cannot be examined on the gravity model parameters or the mode choice utility functions. You do not hold the CTPP credential.”
Gould said: “Has Amara been notified?”
“The NAO auditor emailed her this morning,” Arthur said.
“She has already responded. She confirmed her registration. She sent the complete SATURN scripts and calibration datasets directly to the NAO. She did not contact the planning office before submitting.”
Gould did not say anything for a moment.
He looked at the Green Book submission folder on his desk.
“Gould Demand Forecast.”
He looked at the NAO investigation letter.
“CTPP credential required.”
The London conference hall had been packed with four hundred delegates during his session.
The slides had been prepared by the marketing team in Birmingham — dark blue backgrounds, white lines for the desire flows, the 23% figure highlighted in a glowing yellow circle.
Arthur Vance had spent the hour after the session distributing the print summary brochures to the infrastructure directors who had gathered at the authority’s exhibition stand in the hall.
The exhibition stand had been busy.
The local government delegates had been asking about the WebTAG parameters.
Arthur had handed them the brochure, pointing to the demand forecast section.
“The entire correction is detailed in the Gould forecast,” Arthur had said.
“Dr. Gould resolved the land-use discrepancy that was inflating the ridership.”
“The brochure explains the Green Book appraisal framework.”
Arthur had not read the transport modeling report that was archived in the project directory on the server.
He had not read the certification sheet where CTPP-TD-AS-4416 was stamped in black ink.
He had not read the name: Dr. Amara Sesay.
He sat in his office on the fourth floor of the Municipal Transport headquarters until nine o’clock that night.
The corridors were empty.
The planning staff had left at six.
The infrastructure development team had gone home.
Only the security guard remained on the ground floor, his footsteps echoing faintly in the lobby far below his door.
He had the NAO audit notification letter on his desk.
Beside it lay the printout of the Green Book submission.
The desire line map was printed on page 14 — the blue arrows and red circles showing the 23% demand overstatement.
He had highlighted the eastern corridor zones with a yellow marker when he first reviewed the appraisal copy.
He had been Chief Transport Planner for eight years.
He had signed the Green Book submissions for three separate transit extensions.
Every submission had been compiled under his direction.
That was the structure — the Chief Transport Planner signs the main transmittal sheet.
He was the Chief Transport Planner.
He had understood that authority clearly for eight years, and he had never looked at it closely enough to see the technical boundary that had now appeared in front of him.
He could explain the infrastructure program context.
He could explain the construction timeline.
He could explain the engineering scope — the track geometry, the station designs, the land acquisition schedule.
He had read enough planning summaries to speak confidently about travel demand forecasting and cost-benefit ratios in front of DfT technical panels.
He could not explain the gravity model distribution parameters.
If the NAO panel asked him: how did you calibrate the mode choice utility functions to adjust for the competing retail zones?
He would have no answer.
If they asked: what was the specification of the multinomial logit model used to generate the transit travel probabilities?
He would have no answer.
If they asked: why is the journey time coefficient of negative zero point zero eight the correct representation of travel behavior, and how did you estimate the income-specific value of time?
He had no answer.
He could not defend WebTAG four-stage mode choice calibration he did not conduct.
CTPP-TD-AS-4416 was on the original Statistical Analysis Plan.
CTPP-TD-AS-4416 was Dr. Amara Sesay’s registration.
His MCIHT was a highway engineering designation.
He was not a CTPP-accredited transport planner.
He was a planning officer who managed infrastructure programs.
He looked at the submission folder.
“Gould Demand Forecast.”
His name at the top.
Her name in the appendix.
“Transport modelling support: Dr. Amara Sesay, CTPP-TD-AS-4416.”
He had not written the appendix.
His planning team had formatted the submission from his drafts.
He had reviewed it.
He had signed the submission cover sheet.
He had read “Gould Demand Forecast” and understood it as the project’s output — the technical tool his modeling group had deployed — not as a statement about who had calibrated the trip distribution matrices.
He had been wrong to understand it that way.
He had not known he was understanding it wrongly until this night.
—
The specific moment was a Tuesday afternoon in March, eighteen months ago.
He had been in his office at his desk.
He had the desire line map in his hand — she had just placed it on his desk.
She had said: “The four-stage WebTAG model is the basis of the conditional funding path.”
He had said: “This is the honest forecast the Green Book appraisal needs.”
He had been looking at the blue arrows.
He had been calculating — without calculating formally — how the revised forecast of forty-one thousand two hundred boardings would support the conditional filing, how the value-for-money commitment reviews would be structured, how the Phase III trial’s passenger data would be integrated into the new pathway.
He had understood “the honest forecast the Green Book appraisal needs” as a planning milestone.
He had not understood, in that moment, that identifying the Local Plan land-use conflict and the 23% ridership correction — the work that changed the project’s funding pathway — was not planning framework execution.
He had not understood that the transport modeling correction was itself the discovery.
He had never examined whether “the execution was the invention.”
He had never examined whether the four-stage model was the output of his planning program or the product of her professional expertise.
He had said: “Good work, Amara.”
He had set the map back on her desk.
She had left his office.
He had the modeling report in the submission file.
He had scheduled the design reviews for the next project stage — twelve months away.
He had not escalated the model validation to immediate independent audit.
He had understood “forty-one thousand two hundred boardings” as a model parameter.
He had understood it within the context of transport planning reports.
The NAO value-for-money audit had been opened eighteen months after that conversation.
He picked up his phone.
He opened the planning draft folder.
He typed her name.
“Statistical Analysis Plan by Dr. Amara Sesay, CTPP, CTPP-TD-AS-4416.”
He was beginning to understand that there had been a specific moment when he could have looked at this directly.
That he had been holding the desire line map in his hand when that moment arrived.
That he had said “good work, Amara” instead.
The National Audit Office investigation hearing was held at the NAO headquarters in London on a Thursday morning.
The room was large, formal, and quiet.
Six chairs on one side of the oak table for the NAO senior audit panel.
Two chairs on the other side for the technical witnesses.
The panel consisted of Dr. Heather Lam as the lead auditor, two DfT Audit Assessors, and the committee secretary.
Gould sat in the row of observer chairs along the wood-paneled wall.
He had said to the panel when the session opened: “Dr. Sesay is the CTPP-accredited transport planner who designed the demand model. The technical modeling questions are for her.”
He had stepped back.
He sat down.
He did not speak again.
Amara was at the witness table.
She had brought the desire line map from the project tube.
She unrolled it flat on the table, directly beside the unsigned DfT technical declaration form that the NAO audit team had printed from the Green Book submission folder.
She weighted the corners of the A3 map with two heavy document folders.
The blue arrows.
The red circles.
The red “Local Plan conflict — competing retail sites” annotation in her handwriting.
Beside it, the blank signature line on the declaration form.
Dr. Lam looked at the map.
She said: “Dr. Sesay. For the audit record, please confirm the professional accreditation under which the Eastern Metro demand model was certified.”
“Chartered Transport Planning Professional,” Amara said.
“CTPP-TD-AS-4416.”
“The CTPP accreditation is the recognized UK standard for transport planners who design and certify multi-modal travel demand models for major business cases.”
“The certification is logged in the SATURN model metadata and certified in the report index.”
Dr. Lam wrote in her file.
The committee secretary typed.
Dr. Lam said: “Please explain the modeling mechanism that corrected the 23% ridership overstatement.”
“The original business case model was biased because it used a static gravity model based on an outdated land-use baseline,” Amara said.
“The gravity model assumed that trip generation from the eastern suburbs would distribute to the rail extension transit corridor.”
“Our model updated the trip distribution matrix using the adopted Local Plan land-use database.”
“The Local Plan had committed three major retail development zones on the eastern boundary in 2022.”
“These retail parks capture trips locally, reducing the residential trip length.”
“We calibrated the mode choice utility functions using observed smart card tap logs to estimate the trip distribution.”
“The revised model shows that twelve thousand daily trips are captured by local highway travel, not the rail metro extension.”
“The ridership forecast was reduced from fifty-three thousand six hundred to forty-one thousand two hundred.”
“The 23% reduction represents the land-use conflict resolved by this methodology.”
The first DfT Audit Assessor said: “The original Green Book submission was certified under Chief Transport Planner Dr. James Gould’s authority. On what basis do you say a highway engineering credential is insufficient to certify this WebTAG model?”
“DfT Green Book guidelines require the modeller of record to hold the relevant professional accreditation for complex multi-modal demand appraisals,” she said.
“Dr. Gould is a highways engineer.”
“His MCIHT qualification covers highway capacity, corridor design, and construction management.”
“It does not cover four-stage demand modeling, gravity model trip distribution, or mode choice utility calibration.”
“These tasks require verification of the matrix balancing parameters, calibration of the generalized cost functions, and estimation of the income-specific value of time.”
“These tasks fall under the discipline of Chartered Transport Planning.”
“Dr. Gould did not design the SATURN models.”
“He did not write the calibration scripts.”
“He does not hold the CTPP credential.”
“He cannot certify the demand forecast.”
“The guidelines require the CTPP accreditation for this submission.”
The second DfT Audit Assessor said: “Can you verify that the trip distribution gravity model did not violate the matrix convergence parameters?”
“Yes,” Amara said.
“The gravity model convergence requires that the row and column balancing factor delta remains below zero point zero one.”
“We verified this by plotting the convergence distribution.”
“The balancing delta was zero point zero zero four.”
“There were no extreme zone anomalies that would destabilize the trip matrix.”
“The balancing parameters are detailed in Appendix B of the documentation package.”
“The convergence checks were completed prior to running the final traffic assignment.”
The first DfT Assessor said: “The audit panel must confirm if there was any selection bias in the travel survey data. How did you calibrate the mode choice generalized cost coefficients?”
“The coefficients were calibrated using observed smart card data and local travel surveys,” she said.
“We included in-vehicle travel time, out-of-vehicle walking time, fare structures, and highway toll penalties.”
“The calibration was pre-specified in the modeling methodology before the model run.”
“We did not adjust the coefficients post-hoc to manipulate the ridership forecast.”
“The mode choice model is reproducible.”
“The scripts are in the database.”
Dr. Lam looked at the map.
She looked at the red circles.
She looked at the red text.
She said: “Dr. Sesay. Your CTPP registration and your WebTAG model methodology are the technical foundation of this investigation.”
The audit record that the secretary was building read: “CTPP Transport Planning Professional: Dr. Amara Sesay, CTPP-TD-AS-4416. Four-stage WebTAG model. 41,200 vs 53,600 forecast. SAP certified by Dr. Sesay. Submission amended to name Dr. Sesay.”
—
There were three other authority executives in the room as observers.
The first — a woman in her forties who was the authority’s Director of Risk Management — looked at the desire line map on the table.
She had seen many DfT audits.
She had never seen one resolve a land-use conflict issue this cleanly.
She leaned forward and took a photograph of the map with her phone.
She sat back.
The second observer — a male transport director, late fifties — had read the “Gould Demand Forecast” when it was submitted.
He had noted “Gould” as the certifying officer.
He was now looking at Dr. Amara Sesay answering the modeling questions.
He wrote the CTPP-TD-AS-4416 registration number in his notebook.
Arthur Vance, the Director of Infrastructure Planning, was in the observer row next to Gould.
He had been the one who had distributed the “Gould breakthrough” booklet.
He was watching Dr. Sesay explain the generalized cost calibration.
He did not look at Gould.
—
After the audit hearing closed, Gould walked out to the taxi stand.
He did not wait for Amara.
He called her that evening.
“The audit outcome is satisfactory,” he said.
“Your WebTAG model was the technical foundation.”
“I’ve already filed the amended Green Book submission — your name and CTPP registration going forward.”
“And I’m implementing a department protocol requiring CTPP-registered named authorship on all DfT submissions involving complex demand modeling.”
She said: “The four-stage methodology was complete.”
He said: “Yes.”
A moment of silence.
He said: “Good work, Amara.”
She said: “Yes.”
She rolled the desire line map.
She put it back in the project tube.
The second time he had said “good work.”
The first DfT Assessor reviewed the methodology report.
He looked at the trip attraction variables.
“Dr. Sesay,” he said.
“The WebTAG guidelines require full accounting of any trip rate adjustments.”
“Did the competing retail zones alter the trip generation rates in the adjacent zones?”
“No,” Amara said.
“We held the trip generation rates constant to isolate the trip distribution effect.”
“The trip attraction parameters at the retail zones were estimated using a gravity function based on retail floor space.”
“The travel cost matrices were updated to reflect the new local destinations.”
“This adjustment ensures that the travel demand is redistributed, not artificially inflated by increased trip generation.”
“The trip distribution runs were verified by the independent modeling audit.”
“The statistical proof is in Appendix D of the submission file.”
He nodded.
He noted the response on his sheet.
The secretary’s typing was a steady clicking in the background.
Dr. Lam said: “The NAO value-for-money record will be updated to include the following statement: ‘The travel demand forecast described in the Green Book appraisal is based on, and fairly represents, the four-stage transport model certified by Dr. Amara Sesay, a Chartered Transport Planning Professional.'”
“Is that statement acceptable to you, Dr. Sesay?”
“Yes,” Amara said.
“That statement is correct.”
Six weeks after the audit hearing, she was back in her transport modeling office.
It was a new project — a light rail feasibility study for the western docks, a different corridor, a different mode choice structure, a different land-use baseline.
Kofi was at the smart card data processing station, organizing the new network’s travel survey dataset — cleaning the zone files, setting up the transit matrices, checking the boarding records.
The project tube was on the shelf behind her desk.
She had not moved it since she had returned from London.
She had used the desire line map every morning.
She took the tube from the shelf and unrolled it on her desk — the blue arrows, the red circles, the red “Local Plan conflict” annotation.
She used it as a calibration reference: comparing the new corridor’s trip distribution pattern against the Eastern Metro desire line baseline, confirming that the origin-destination zones were appropriately aligned before beginning the new SATURN calibration.
The new corridor’s study area had competing bus routes, but the gravity parameters were comparable.
The land-use constraints were different.
But the four-stage modeling logic was the same.
The trip distribution matrices.
The generalized cost functions.
The convergence criteria.
The NAO investigation record was filed in the public archive.
“CTPP Transport Planning Professional: Dr. Amara Sesay, CTPP-TD-AS-4416. Four-stage WebTAG demand model. 41,200 vs 53,600 forecast. SAP certified by Dr. Sesay. Submission amended to name Dr. Sesay as certifying transport planner. NAO value-for-money audit completed. Efficacy reanalysis certified under CTPP registration rules.”
That record was permanent.
Kofi said, from the data station: “CTPP-TD-AS-4416 is in the NAO record.”
She said: “Yes.”
Kofi said: “The red annotation.”
She said: “The red annotation is in the record.”
Kofi looked at the map.
He went back to the dataset formatting.
—
The original DfT project register was still on the DfT website — a ninety-two-page PDF document detailing the funding decision, containing “Gould’s demand forecast” in the technical reference section.
The document had been downloaded by hundreds of transport directors and local government planners since the funding was announced.
It was not recalled.
It remained in the public regulatory archive.
She had the DfT project reference number: DfT/T/C/005824.
The amended submission was in the NAO file.
The public project register page had not been changed.
Both were permanent records.
The authority’s new CTPP-equivalent modeling protocol had arrived as a PDF attachment from the planning office: “Protocol STAT-DFT-2024-12: CTPP registration certification requirement for all regulatory Green Book submissions involving complex travel demand modeling.”
She had read it.
She had filed it in the department files.
—
The new model brief had arrived from Gould on a Friday afternoon.
Subject: “WebTAG model — Western Docks — Feasibility Study.”
It read: “WebTAG model — Dr. Amara Sesay, CTPP lead.”
She had opened it.
She had begun the model design for the new transit corridor.
Kofi had the first batch of covariates ready.
He said: “The travel survey database is loaded.”
She loaded the model scripts.
She looked at the blank axes on her monitor — the space where the trip distribution curves would appear when the SATURN runs finished.
She looked at the map on her desk.
She looked at the blue arrows.
The Western Docks light rail project was statistically complex.
The mode share was influenced by both journey time and time-varying highway tolls.
Commuters who traveled during peak hours switched modes earlier than those who traveled during off-peak times without toll penalties.
This was not a simple transit crossover problem.
Tolls and congestion acted as joint time-varying factors.
If the tolls were associated with peak congestion, the transit mode share would be high, but the trip distribution would be skewed.
If the tolls were independent of congestion, the calibration would be straightforward, but the generalized costs would be highly variable.
She needed to specify these joint generalized cost models.
She would run twelve hundred simulations across the ten main zone centroids to check the model’s convergence.
Kofi would handle the zone coding, the network preparation, and the matrix balancing.
It would take four weeks of model design.
The office was cool.
The computer fans hummed behind her desk, maintaining a steady temperature as the processor handled the assignment runs.
The server connection status indicator flashed green, regulating the data transfer from the transport database in Leeds.
She looked at the blank software screen.
The calibration of the gravity parameters for the joint factors was the critical gate.
Tolls and congestion had overlapping timelines — both occurred during peak travel hours, but tolls had distinct pricing structures that the model must separate from the physical congestion delays.
She had spent three days calibrating the cost parameters using travel time indicators from the pilot study’s reference database.
The model profile was saved as “Docks-Transit-v1.4.lib.”
She looked at the desire line map on her desk.
The Eastern Metro project had been simple compared to this.
The previous project had a single land-use conflict, easily adjusted by the Local Plan dataset.
The joint travel cost factors would require complex multi-class assignment models and careful calibration of the toll-congestion ratios.
But the physical reality remained.
The travel flows on the screen were a reflection of the actual travel choices.
The survey data did not know who had signed the DfT declaration.
The travel probability would change according to the generalized cost, regardless of whether Gould’s name or her name was printed on the Green Book submission.
The ridership was locked in the travel survey database.
She looked at the map.
The blue arrows.
The red circles.
She unrolled the map.
She pointed to the red.
