The European Union should invest in the road infrastructure between Poland and Lithuania in my opinion. There are but three road connections along the approx. 66 km wide border, one primary road and two lesser ones of which one is an easily blocked route through woodland close to the border of Belarus.
Only offroad-capable vehicles such as modestly-laden (not maximum nominal payload) 8x8 lorries could traverse some other routes along paths otherwise used by agricultural vehicles - and those paths would neither sustain much traffic before being ruined nor be reliable options in thawing weather.
A defence of the Baltic countries - a quick "counter-concentration" to deter aggression as is still NATO doctrine as far as I know or a wartime influx of forces and supplies - would depend on these roads. Their capacity simply isn't satisfactory, particularly if you consider the possible effects of hostile actions.
The European Union could change this with a modest budget, maybe € 100 million. Additional roads could be added and existing roads could be widened or paved. Drainage ditches, ramparts or walls of any kind that would impede driving around craters or wrecks on the road could be avoided at little to no extra costs. It only takes that the road engineers keep in mind the military logistics relevance and requirements.
An improvement of this infrastructure would help the region economically. It could easily be included in the general spending on infrastructure projects within the EU budget and wouldn't even be noticed as a burden throughout the EU. It would still solve about half of the Baltic defence problems and challenges in my opinion.
One could disagree, and point out how two Russian divisions made it into South Ossetia through a single tunnel a few years ago. Isn't even a single road enough?
Well, as I wrote years ago: "A few B-2 bomber sorties could have demolished an entire Russian division on a valley road during the South Ossetia War, for example. And the road as well." Imagine a mere 100 GLONASS- or backup INS-guided bombs or heavy artillery rockets with impact points spaced by 100 m. With point detonation fuses this could ruin an entire division on its administrative march. With delay fuses this could create dozens of craters that would overwhelm the capabilities of organic engineers to sustain mobility. A day or more would be lost until the craters are filled up, circumvented over mat-reinforced ground or bridged - and this could be crucial both in planning and reality. Finally, this kind of damage could be done to a single road again and again every few hours or so - by a single Smerch battery with suitable ammunition.
Meanwhile, Poland could stock up on Leguan bridge segments (or equivalents), mat laying vehicles and mats (example German FSG, but there are several such systems), low light vision-compatible route markers, ready-to-use durable fascines and possibly quick-dry cement and its specific tools.
None of this would be sexy, none of this would excite many vocal fanbois, none of this would be attractive to military top brass, none of this would improve a "defence" politician's career, none of this would appear in a great power's "defence review", none of this would make it into national political debate other than in Lithuania, none of this would excite EU bureaucrats who tend to focus on other regions - but all of this would greatly enhance Russian respect for Western seriousness and preparedness regarding Baltic security ... at the price of maybe a single sexy "Typhoon" fighter-bomber.
P.S.: The road-laying systems are as far as I know mostly of relevance for improvised roads to pontoon bridge and other mil engineering bridges not connected to the civilian road network. I've never seen a system of this kind optimised for circumventing a damaged road section. They appear to be optimised for straight improvised roads only. Maybe military engineers trust their ability to fill up or bridge craters more than I do or here's a possible capability optimisation niche (market niche).
A Smerch rocket and equivalent KAB-250-SE bomb are capable of producing a crater with 5-10 m diameter if the descent angle is fine. Bigger bombs usually produce bigger craters, but dedicated anti-runway bombs could create craters very efficiently.