More than 50 groups connected to transport and public health have urged the transport secretary, Heidi Alexander, to set specific targets for levels of walking and cycling in England, warning that plans as they stand are too vague.

A letter from groups including British Cycling, Cycling UK, the National Trust and the British Medical Association says the government’s proposals for active travel must “move from good intentions to a clear, long-term, fully deliverable national plan comparable to other strategic transport programmes”.

Transport planners for modes such as road and rail have the confidence of established funding and plans setting out objectives over decades, the groups point out, contrasting this with what they say remains an often short-term and piecemeal approach to active travel despite this making up a third of all trips.

The government has promised unprecedented levels of funding for walking, wheeling and cycling. Ministers are consulting on the third cycling and walking investment strategy (CWIS3), which promises a “fundamental shift” in how active travel is treated.

Some parts of the active travel world not connected to the letter have expressed concern that the Department for Transport (DfT) under Alexander is less engaged with the subject than it was under her predecessor, Louise Haigh, and that momentum is being lost.

In contrast, the letter, which is also signed by representatives of the Association of Directors of Public Health, Ramblers and three of England’s regional active travel commissioners, is not directly critical and is trying to engage positively while making it plain that more should be done.

Saying that CWIS3 should put active travel on an equal footing with roads and rail, it argues for clear targets that should be met by 2030, the period covered by the strategy, including that by then 50% of trips of less than five miles in towns and cities should be walked, wheeled or cycled.

The letter warns CWIS3’s proposed objectives, which include “ensuring people are safe to travel actively”, are “open to interpretation and not measurable”.

It goes on: “Other transport modes operate with predictable five-year cycles and transparent progress reporting; active travel can and should match that level of maturity. Long-term targets, with interim reviews, will give local leaders the confidence to plan networks, support larger projects, unlock private sector investment and demonstrate how devolved investment and progress contribute to national missions.”

There should also be a national strategy for active travel networks, it adds, saying that while things like walking and cycling routes should be designed locally, they also need to be “coherent, connected, consistent and accessible to all” and should link to key destinations such as rail and bus stations, hospitals and schools.

The letter says: “Without reliable, safe, joined-up routes to walk, wheel and cycle, people cannot reasonably be expected to travel actively, impacting health, opportunities and independence from childhood to older adulthood.”

Xavier Brice, the chief executive of Walk Wheel Cycle Trust, one of the signatories, said CWIS3 was originally intended to shift active travel “from stop-start funding and priorities to a long-term framework for investment and change. However, it has never quite lived up to this promise.”

Without targets, he said, even the existing target in the strategy for active to be a safe, easy and accessible option for everyone by 2035 “is unachievable under the current proposals”.

A DfT spokesperson said: “We have committed £616m to Active Travel England up to 2030 so that local authorities can deliver walking and cycling infrastructure across the country, with the aim that by 2035 walking, wheeling and cycling is easy, safe, and accessible for everyone. We have just launched a consultation on how we can achieve this, and we encourage stakeholders and the public to have their say.”