Various things have been proposed as alternatives to LTNs. Some of the proposals are complementary to low traffic neighbourhoods rather than alternatives to them. Others rely on technology that doesn't exist, funding that doesn't exist, or are otherwise fantastic. None will achieve the core active travel and liveability goals of a low traffic neighbourhood.
Improve cycling infrastructure
Putting cycle tracks onto main roads and making the main road junctions walking- and cycling-friendly is a worthy goal, but this would be a complement to LTNs rather than an alternative to them. Improving those junctions will also require space and time to be reallocated from motor traffic, which will reduce peak motor traffic throughput... And both junction rebuilds and main road cycle tracks are really expensive — on the order of £1 million per kilometre, or even more.
Side streets such as Rymer's Lane or Crescent Rd don't have room for cycle lanes, even if all on-street parking were removed (which seems likely to be extremely unpopular) and previous attempts at traffic calming have in some ways made them worse for cycling. Looking at either the UK Cycle Infrastructure Design guidance or the Dutch CROW manual, the only way to make cycling accessible on these roads is to reduce motor traffic volumes. And there is no funding for any of this (keeping in mind that a single signalled pedestrian crossing would cost more than the three Cowley LTN schemes put together).
Put speed cameras and/or traffic calming everywhere
The county can't even afford to put speed cameras on Oxford's main roads, let alone on every street. And even really aggressive traffic calming doesn't help when traffic volumes are too high: Rymer's Lane is an obvious example, and children regularly cycle on the pavement on Howard St. The problem on streets like this is too much motor traffic, with speeding a secondary problem, in some places and at some times.
Deploying and mandating Intelligent Speed Assistance in new vehicles is a great idea, but would take a decade and is not something the county has any control over.
Make the buses free
Cheaper and better public transport would be good. But it wouldn't address the goal of enabling walking and cycling on local streets — including the trips needed to get to bus stops. (Few primary school children catch buses to school, so better buses wouldn't help there at all.)
Also, providing free buses in Oxford might cost on the order of £20 million a year. That can't conceivably be funded out of council budgets. (Bringing the buses under local authority control would have benefits, but would have minimal effect on fares.)
do School Streets schemes instead
School Streets schemes only reduce road danger in the area immediately outside schools. That is important, but will have relatively little effect on enabling walking and cycling unless coupled with the broader road danger reduction of LTNs.
So why not extend the School Streets schemes to cover the whole LTN area, and use time-restricted camera-controlled modal filters everywhere? First of all, Oxfordshire doesn't have the power to use cameras for that except on bus routes (London boroughs and Welsh LAs do). And cameras cost a lot more than planter boxes.
In any event, replacing the LTNs with pseudo-LTNs that only operated (say) 7.45-9.15 and 2.30-4.30 on weekdays in term-time would seem likely to retain most of their negative effects while only achieving a fraction of their positive ones. It would cripple the longer-distance cycle routes that run through the LTNs, and would give up on children or less confident adults who wanted to walk or cycle to the park, or to the shops, or to visit friends — enabling children to walk or cycle to school is important, but is hardly the only active travel goal. A scheme like this would also create a lot of anger from people getting fined after forgetting it was school term or getting confused about the times.