It is known that $I(p) = \sqrt { \frac {4 \mathrm{i} p} {\pi}}\int \limits _a ^b \mathrm{e}^{- \mathrm{i} p x^2} \varphi (x) \mathrm{d}x$ is a bounded smooth function on $(0,\infty)$ with $\lim \limits _{p \to \infty} I(p) = 1$.

Does one have an even better understanding of $I(p)$ as $p$ aproaches $\infty$? You see, I am not only interested in its limit, but also in $how$ it tends to it. Something like the "dominant" part of $I(p)$, or some tight bound (of it or its absolute value), or any other approximation would be of help.