Commuter journeys
Data on commuter journeys for work are gathered in the Census. The table below summarises 2011 Census data for the Cambridge region on the number of people commuting into Cambridge daily for work from outside of the City, and the number of residents commuting for work within the City daily (Source: 2011 Census / Office of National Statistics – data kindly supplied by M. Schofield). These data have been projected to 2031 based on an expected 18% growth over this period.
Taking peak hours as 07:00 – 09:30, there is a 2.5 hr window in which the majority of morning commuters will be traveling. This allows calculation of an approximate number of commuters per hour traveling for work at peak. The number of commuter journeys per day is double the total number of commuters (one journey each way per commuter). The peak hour rate of commuters per hour is assumed to be the same between 16:00 – 18:30 in the evening as in the morning.
Table 1. Number of commuters and journeys for work per day in Cambridge
| Year | Commuters INTO Cambridge /day | Commuters WITHIN Cambridge /day | Total commuters /day | Total commuter Journeys /day |
Total commuter Journeys /hr (peak) |
| A | B | C=A+B | D=C*2 | E=C/2.5 | |
| 2011 | 51 000 | 44 000 | 95 000 | 190 000 | 38 000 |
| 2031 | 60 180 | 51 920 | 112 100 | 224 200 | 44 840 |
Non-commuter journeys
In practice, people make journeys for many reasons other than commuting to or from work. The Cambridge Access Study (Mott Macdonald 2015 Cambridge Access Study: p.10) reported the Cambridgeshire Transport Innovation Fund (TIF) 2009 study finding that ~23% of daily journeys are by commuters for work, with the remaining 77% comprising journeys for shopping, education, and health (the latter presumably includes other reasons such as leisure, etc). Based on this split, Table 2 shows the number of journeys made for reasons other than commuting at peak. Table 2 represents an over-estimate because non-commuter journeys made by residents outside of Cambridge City are included, and some of those journeys will not be into Cambridge City. However, many non-commuter journeys made by non-residents of Cambridge City will be to and from Cambridge, so for the moment it seems a reasonable approximation until all data are assembled.
Table 2. Number of non-commuter journeys per day in Cambridge.
| Year | Non-commuter1 Journeys TO /d | Non-commuter1 Journeys FROM /d | Non-commuter1 Journeys TOTAL /d | Non-commuter Journeys /h2 |
| A=(Table1C/0.23)-Table1C | B=A | C=A+B | D=C/12 | |
| 2011 | 318 043 | 318 043 | 636 087 | 53 007 |
| 2031 | 375 291 | 375 291 | 750 583 | 62 549 |
1. Assumes non-commuters = 77% of total journeys
2. Assumes 12 h day and equal spread of journeys over day
Non-commuter journeys are thus 636K per day in 2011 and are projected to be around 750K per day in 2031.
Total journeys
To estimate total potential demand on public transport, we need to add the demand for other travel purposes to commuter demand (Table 3).
Table 3. Total number of journeys per day in Cambridge.
| Year | Total journeys commuters /d | Total journeys non-commuters /d | Total journeys /d | Total journeys /h (peak)1 |
| A=Table1D | B=Table2C | C=A+B | D=Table1E+Table2D | |
| 2011 | 190 000 | 636 087 | 826 087 | 91 007 |
| 2031 | 224 200 | 750 583 | 974 783 | 107 389 |
1. Number of non-commuter peak hour journeys is assumed to be at the same rate per hour as non-peak.
Public transport to satisfy demand
Because the Census also gathers data on mode of transport commuters use, we may calculate from the tables above how many people travel by public transport, which in Cambridge is principally by bus.
Around 8% of commuters for work traveled by bus in 2011 (~7% for commuters within Cambridge and ~9% for commuters from outside)(Sources: Transport Strategy for Cambridge and South Cambridgeshire 2014 and 2011 Census / Office of National Statistics – data kindly supplied by M. Schofield). This percentage has remained more or less constant since 2002. We can calculate the number of buses per hour required to satisfy demand from commuters to work for bus travel into and within the City of Cambridge (Table 4).
Table 4. Number of commuters journeying to work by bus into & within Cambridge at peak and number of buses required
| Year | % by bus | pax /h | Pax /bus | Buses /h | Buses per 5 mins |
| A | B=(Table1E*A)/100 | C | D=B/C | E=D/12 | |
| 2011 | 8 | 3040 | 75 | 41 | 3 |
| 2031 | 15 | 6726 | 75 | 90 | 7 |
| 2031 | 20 | 8968 | 75 | 120 | 10 |
| 2031 | 25 | 11 210 | 75 | 149 | 12 |
Based on 8% of commuters per hour at peak, the number of buses required to service only commuters for work in 2011 (assuming all buses were full double-deckers) would be around 41 buses per hour, or about 3 buses every 5 minutes. This provides a rough approximation of the number of buses required to satisfy present commuter demands at a low level of public transport adoption. In practice, bus occupancy rates are much less than 100%. Dept for Transport data for 2014/15 show that average bus occupancy for metropolitan areas in England outside of London is only ~10.4% (equivalent to 9 buses out of every 10 running empty). Even taking into account that peak hour buses run nearer full, the number of buses required to meet the demand above would, in practice, be much greater than shown in Table 4.
Let us assume that by improving bus services we can double the numbers of commuters choosing to take the bus to 15% by 2031. The number of buses required to service only commuters for work at that time, taking into account an 18% increase in commuters, would rise to 90 per hour, or around 7 buses every 5 minutes. Let us further assume that the ‘target of low ambition’ of 15% is raised to 20% or 25%, the number of buses required would rise to between 120 to 149 buses per hour running full, or 10-12 buses every 5 mins. Given occupancy rates are most unlikely to be full, the actual numbers of buses would need to be considerably higher to transport the required number of commuters.
Compared to many other European cities, 8% represents a poor modal share for public transport. In the context of Cambridge, people’s choices to commute by bus has remained unchanged at around 7-8% over the last 10-15 years, and a target of 15% seems an optimistic and even unrealistic outlook.
Recalling that commuters only account for about 23% of journeys per day, in reality there would be a need for substantially more buses per hour than these figures suggest to satisfy demand. Table 5 shows the number of buses required based on both commuter and non-commuter journeys.
Table 5. Number of journeys by bus into & within Cambridge at peak by all passengers and number of buses required
| Year | % by bus | Pax /h | Pax /bus | Buses /h | Buses per 5 mins |
| A | B=(Table3D*A)/100 | C | D=B/C | E=D/12 | |
| 2011 | 8 | 7281 | 75 | 97 | 8 |
| 2031 | 15 | 16 108 | 75 | 215 | 18 |
| 2031 | 20 | 21 478 | 75 | 286 | 24 |
| 2031 | 25 | 26 847 | 75 | 358 | 30 |
Smarter Cambridge Transport used an alternative method to estimate the number of buses entering the city centre in 2031. Based on projections made in a recent study (WSP Parsons Brinkerhoff 2015. Milton Road & Histon Road corridors draft options report. Report No 70012012-002), with 51 buses /h and 22 buses /h predicted for Milton Road and Histon Road respectively, Smarter Cambridge Transport estimated the number of buses likely to be incoming on all radial routes to derive a figure of 267 buses /h, or 22 buses every 5 mins, conveying ~20K pax /h. These projections are similar to those in Table 5 with a 15-20 % modal share for public transport, bearing in mind that Table 5 assumes all buses are full double-deckers. The WSP report does not include the basis of their projections, although the data are presented as the “potential future frequency and services needed to support future growth” (p.10).
If a large proportion of these buses were making journeys into the City centre (as the majority of bus services currently do, specifically Drummer Street, Regent Street, Bridge Street, Cambridge Central Rail station etc), then it is clear that space to park buses for passenger exchange will be at a premium. Let us assume a ‘residence time’ of a full bus exchanging passengers is (optimistically) around 5 mins. With between 10-15 buses arriving at central bus stops every 5 minutes, it would seem that perfect passenger exchanges would be required by every bus to avoid queues rapidly building up where buses drop-off / pick-up passengers.
These exploratory scenarios based on Cambridge commuter data raise serious questions about whether a bus-based public transport network operating on Cambridge’s narrow mediaeval network of streets is likely to be either practical or sustainable. Moreover, the number of buses potentially required has major implications for inner city heritage, including globally outstanding landscapes such as The Backs and for many colleges. While these calculations are based on partial data and a number of assumptions, the close correspondence in results between two independent methods as used above and by WSP Parsons Brinkerhoff and Smarter Cambridge Transport lend support to the conclusion that the number of buses would need to be substantial in order to meet projected demand based on current population forecasts.
In reality, the Drummer Street bus terminal already experiences congestion with current demand, and the current space will be insufficient to accommodate the number of buses required in a practical operation in 2031. While it might be possible to distribute bus stops less centrally to help manage pressures on space, this would have a severe impact on passenger convenience and journey times (people would need to walk from bus stops to the City centre). It might also be possible to construct an underground bus terminal, as suggested in Cambridge Futures 2, although this approach alone would not address the significant impacts of the large number of bus movements on the inner city approach streets. Bus tunnels (again as suggested in Cambridge Futures 2) might be an option to address these difficulties, although only single-decker buses are feasible in tunnels, which in itself has major implications for the number of buses required to meet projected demand.
The discussion above raises many questions about the appropriateness of a bus-based approach to solutions for public transport in Cambridge over the medium- to longer-term. The values of the historic city, with heritage of outstanding global importance, and a remarkable architecture and landscape need to be taken into account. In addition, long-term objectives related to achieving greater modal share by public transport need more detailed consideration, as does the need to consider approaches that will help to achieve a more sustainable city.
Visitor demand
None of the above calculations take into account the demand for public transport by visitors, since this lies outside Census and resident survey data. Cambridge is one of the most popular tourist destinations in the United Kingdom, and approximately 5 million tourists visited the City in 2014, with numbers approaching ~7 million before the COVID pandemic. It is worth reflecting that a larger population will serve to attract more transient visitors, and the extent to which this is being taken into account in demand projections for transport is not fully clear in reports such as the Cambridge Access Study and the WSP report noted above.
Let us assume one in four tourists would use public transport within the City during their visit, with use spread equally over a 12-hour period of every day of the year. The calculation 5M x 0.25 /365 /12 gives us an estimate of 285 tourists per hour using the public transport network. In practice, visitor demand for journeys would tend to peak towards the start and end of the day, with sightseeing during the day. Tourist numbers and patterns of behaviour are a major omission in current demand projections and also need to be taken into account.
